Thames House, London
|Formed||1909 as the Secret Service Bureau|
|Jurisdiction||Her Majesty's Government|
|Headquarters||Thames House, London, UK|
|Motto||Regnum Defende ("Defend the Realm")|
|Employees||4,053 (31 March 2016)|
|Annual budget||Single Intelligence Account (£3.02 billion in 2017–2018 financial year)[nb 1]|
The Security Service, also known as MI5 (Military Intelligence, Section 5), is the United Kingdom's domestic counter-intelligence and security agency and is part of its intelligence machinery alongside the Secret Intelligence Service (MI6), Government Communications Headquarters (GCHQ) and Defence Intelligence (DI). MI5 is directed by the Joint Intelligence Committee (JIC), and the service is bound by the Security Service Act 1989. The service is directed to protect British parliamentary democracy and economic interests, and counter terrorism and espionage within the UK.
The Security Service comes under the authority of the Home Secretary within the Cabinet. The service is headed by a Director General at the grade of a Permanent Secretary of the British Civil Service who is directly supported by an internal security organisation, secretariat, legal advisory branch and information services branch. The Deputy Director General is responsible for the operational activity of the service, being responsible for four branches; international counter-terrorism, National Security Advice Centre (counter proliferation and counter espionage), Irish and domestic counter-terrorism and technical and surveillance operations.
The service is directed by the Joint Intelligence Committee for intelligence operational priorities. It liaises with SIS, GCHQ, DIS, and a number of other bodies within the British government and industrial base. It is overseen by the Intelligence and Security Committee of Members of Parliament, who are directly appointed by the Prime Minister, by the Interception of Communications Commissioner, and by the Intelligence Services Commissioner. Judicial oversight of the service's conduct is exercised by the Investigatory Powers Tribunal.
Operations of the service are required to be proportionate and compliant with British legislation including the Regulation of Investigatory Powers Act 2000, the Investigatory Powers Act 2016, the Data Protection Act 2018, and various other items of legislation. Information held by the service is exempt from disclosure under section 23 of the Freedom of Information Act 2000.
All employees of the service are bound by the Official Secrets Act. In certain circumstances, employees can be authorised to carry out activity which would otherwise be criminal within the UK.
The service marked its centenary in 2009 by publishing an official history titled The Defence of the Realm: The Authorized History of MI5 and written by Christopher Andrew, Professor of Modern and Contemporary History at Cambridge University.
The Security Service is derived from the Secret Service Bureau, founded in 1909 and concentrating originally on the activities of the Imperial German government as a joint initiative of the Admiralty and the War Office. The Bureau was split into naval and army sections which, over time, specialised in foreign target espionage and internal counter-espionage activities respectively. This specialisation was a result of the Admiralty intelligence requirements related to the maritime strength of the Imperial German Navy. This specialisation was formalised prior to 1914 and the beginning of World War I, with the two sections undergoing a number of administrative changes and the home section becoming Directorate of Military Intelligence Section 5 (MI5), the name by which it is still known in popular culture.
The founding head of the Army section was Vernon Kell of the South Staffordshire Regiment, who remained in that role until the early part of the Second World War. Its role was originally quite restricted; existing purely to ensure national security through counter-espionage. With a small staff and working in conjunction with the Special Branch of the Metropolitan Police, the service was responsible for overall direction and the identification of foreign agents, whilst Special Branch provided the manpower for the investigation of their affairs, arrest and interrogation.
On the day after the declaration of World War I, the Home Secretary, Reginald McKenna, announced that "within the last twenty-four hours no fewer than twenty-one spies, or suspected spies, have been arrested in various places all over the country, chiefly in important military or naval centres, some of them long known to the authorities to be spies", a reference to arrests directed by the service. These arrests have provoked recent historical controversy. According to the official history of MI5, the actual number of agents identified was 22 and Kell had started sending out letters to local police forces on 29 July giving them advance warning of arrests to be made as soon as war was declared. Portsmouth Constabulary jumped the gun and arrested one on 3 August, and not all of the 22 were in custody by the time that McKenna made his speech, but the official history regards the incident as a devastating blow to Imperial Germany which deprived them of their entire spy ring, and specifically upset the Kaiser.
This view has been challenged by Nicholas Hiley who has asserted that it is a complete fabrication. In 2006 his article "Entering the Lists" was published in the journal Intelligence and National Security outlining the products of his research into recently opened files. Hiley was sent an advance copy of the official history and objected to the retelling of the story. He later wrote another article, "Re-entering the Lists", which asserted that the list of those arrested published in the official history was concocted from later case histories.
MI5 was consistently successful throughout the rest of the 1910s and 1920s in its core counter-espionage role. Throughout World War I, Germany continually attempted to infiltrate Britain but MI5 was able to identify most, if not all, of the agents dispatched. MI5 used a method that depended on strict control of entry and exit to the country and, crucially, large-scale inspection of mail. In post-war years, attention turned to attempts by the Soviet Union and the Comintern to surreptitiously support revolutionary activities within Britain. MI5's expertise, combined with the early incompetence of the Soviets, meant the bureau was successful in correctly identifying and closely monitoring these activities.
In the meantime, MI5's role had been substantially enlarged. Due to the spy hysteria, MI5 had been formed with far more resources than it actually needed to track down German spies. As is common within governmental bureaucracies, this caused the service to expand its role, to use its spare resources. MI5 acquired many additional responsibilities during the war. Most significantly, its strict counter-espionage role blurred considerably. It became a much more political role, involving the surveillance not merely of foreign agents but also of pacifist and anti-conscription organisations, and of organised labour. This was justified through the common belief that foreign influence was at the root of these organisations. Thus, by the end of the World War I, MI5 was a fully-fledged investigating force (although it never had powers of arrest), in addition to being a counter-espionage agency. The expansion of this role continued after a brief post-war power struggle with the head of the Special Branch, Sir Basil Thomson.
After World War I, Kell's department was considered unnecessary by budget-conscious politicians. In 1919, MI5's budget was slashed from £100,000 and over 800 officers to just £35,000 and 12 officers. At the same time, Sir Basil Thomson of Special Branch was appointed Director of Home Intelligence, in supreme command of all domestic counter-insurgency and counter-intelligence investigations. Consequently, as official MI5 historian Christopher Andrew has noted in his official history Defence of the Realm (2010), MI5 had no clearly defined role in the Anglo-Irish War. To further worsen the situation, several of Kell's officers defected to Thomson's new Agency, the Home Intelligence Directorate. MI5 therefore undertook no tangible intelligence operations of consequence during the Irish War of Independence. MI5 did undertake the training of British Army Case Officers from the Department of Military Intelligence (DMI) for the Army's so-called "Silent Section" otherwise known as M04(x). Quickly trained by MI5 veterans at Hounslow Barracks, outside London, these freshly minted M04(x) Army case officers were deployed to Dublin beginning in the Spring of 1919. Over time, 175 officers were trained and dispatched to Ireland. In Ireland, they came under the command of General Cecil Romer and his Deputy, Lieutenant Colonel Stephen Searle Hill-Dillon.
In April 1919, Colonel Walter Wilson of DMI arrived in Dublin to take over the day-to-day management of these 175 Army intelligence officers, and the unit was designated as the "Dublin District Special Branch" (DMI/MO4(x)/DDSB) because it operated exclusively within the confines of the Army's Dublin Military District. Royal Marine Colonel Hugh Montgomery of the Department of Naval Intelligence, was also seconded to Romer's intelligence staff at this time. British Army after-action reports and contemporary accounts indicate that M04(x)/DDSB was considered a highly amateurish outfit. Serious cover constraints, coupled with alcohol abuse and social fraternization with local prostitutes would prove to be the downfall of several of these amateur sleuths. Despite these failings, it was not MI5 but one of Basil Thomson's agents, John Charles Byrnes, a double agent within the IRA, who identified Michael Collins and came close to arranging his capture. Byrnes was discovered as a British spy and executed by the IRA in March 1920.
The intelligence staff of Michael Collins Irish Republican Army penetrated the unit. Using DMP detectives Ned Broy and David Nelligan, Michael Collins was able to learn the names and lodgings of the M04(x) agents, referred to by IRA operatives as "The Cairo Gang". On Bloody Sunday, Collins ordered his Counter-intelligence Unit, The Squad, to assassinate 25 M04(x) agents, several British Courts Martial Officers, at least one agent reporting to Basil Thomson, and several intelligence officers attached to the Royal Irish Constabulary Auxiliary Division, at their lodgings throughout Dublin. Although the shooting of 14 British officers had the desired effect on British morale, in many ways Bloody Sunday was a botched job. Three of Collins's men were apprehended after engaging in a shoot-out on the street, and at least two of the wounded British officers had no connection whatsoever to British Intelligence. Moreover, with MO4(x) having fielded a total of 175 agents of the DDSB, Collins's operation only temporarily slowed British momentum. Within days, the remaining 160-odd M04(x) agents were re-established in secure quarters inside solidly Loyalist hotels in Dublin, from where they continued to pursue Collins and the IRA relentlessly right up until the Truce. In December 1920 the entire DDSB was transferred from British Army Command to civil command under Deputy Police Commissioner General Ormonde Winter, and thereafter was known as "D Branch" within Dublin Castle. By January 1921, the highly experienced MI6 operative David Boyle arrived at Dublin Castle to take over the day-to-day management of D Branch. The unit's former commander, Colonel Wilson, resigned in protest for having had his command taken from him. D Branch thrived under Boyle's leadership. The net impact of Collins's strike of Bloody Sunday, 21 November 1920, was therefore quite negligible—even though the IRA had not gone up against MI5 professionals but instead only a quickly trained outfit of amateur army "D-Listers."
That afternoon, a mixed force of the British Army, the Royal Irish Constabulary, and the Black and Tans retaliated by indiscriminately shooting dead 14 civilians at a Gaelic Football match at Croke Park.
In 1921, Sir Warren Fisher, the Government inspector general for civil service affairs, conducted a thorough review of the operations and expenditures of Basil Thomson's Home Intelligence Directorate. He issued a scathing report, accusing Thomson of wasting both money and resources and conducting redundant as well as ineffectual operations. Shortly thereafter, in a private meeting with Prime Minister David Lloyd George, Sir Basil Thomson was sacked, and the Home Intelligence Directorate was formally abolished. With Thomson out of the way, Special Branch was returned to the command of the Commissioner of The Criminal Investigation Division at Scotland Yard. Only then was Vernon Kell able once again to rebuild MI5 and regain its former place as Britain's chief domestic spy agency.
MI5's decline in counter-espionage efficiency began in the 1930s. It was, to some extent, a victim of its own success. It was unable to break the ways of thinking it had evolved in the 1910s and 1920s. In particular, it was unable to adjust to the new methods of the Soviet intelligence services the NKVD and GRU. It continued to think in terms of agents who would attempt to gather information simply through observation or bribery, or to agitate within labour organisations and the armed services, while posing as ordinary citizens. The NKVD, meanwhile, had evolved more sophisticated methods; it began to recruit agents from within the upper classes, most notably from Cambridge University, who were seen as a long-term investment. They succeeded in gaining positions within the Government (and, in Kim Philby's case, within British intelligence itself), from where they were able to provide the NKVD with sensitive information. The most successful of these agents—Harold "Kim" Philby, Donald Maclean, Guy Burgess, Anthony Blunt and John Cairncross—went undetected until after the Second World War, and were known as the Cambridge Five.
Second World War
MI5 experienced further failure during the Second World War. It was chronically unprepared, both organisationally and in terms of resources, for the outbreak of war, and utterly unequal to the task which it was assigned—the large-scale internment of enemy aliens in an attempt to uncover enemy agents. The operation was poorly handled and contributed to the near-collapse of the agency by 1940. One of the earliest actions of Winston Churchill on coming to power in early 1940 was to sack the agency's long-term head, Vernon Kell. He was replaced initially by the ineffective Brigadier A.W.A. Harker, as Acting Director General. Harker in turn was quickly replaced by David Petrie, an SIS man, with Harker as his deputy. With the ending of the Battle of Britain and the abandonment of invasion plans (correctly reported by both SIS and the Bletchley Park Ultra project), the spy scare eased, and the internment policy was gradually reversed. This eased pressure on MI5, and allowed it to concentrate on its major wartime success, the so-called "double-cross" system.
This was a system based on an internal memo drafted by an MI5 officer in 1936, which criticised the long-standing policy of arresting and sending to trial all enemy agents discovered by MI5. Several had offered to defect to Britain when captured; before 1939, such requests were invariably turned down. The memo advocated attempting to "turn" captured agents wherever possible, and use them to mislead enemy intelligence agencies. This suggestion was turned into a massive and well-tuned system of deception during the Second World War.
Beginning with the capture of an agent named Owens, codenamed Snow, MI5 began to offer enemy agents the chance to avoid prosecution (and thus the possibility of the death penalty) if they would work as British double-agents. Agents who agreed to this were supervised by MI5 in transmitting bogus "intelligence" back to the German secret service, the Abwehr. This necessitated a large-scale organisational effort, since the information had to appear valuable but actually be misleading. A high-level committee, the Wireless Board, was formed to provide this information. The day-to-day operation was delegated to a subcommittee, the Twenty Committee (so called because the Roman numerals for twenty, XX, form a double cross).
The system was extraordinarily successful. A postwar analysis of German intelligence records found that of the 115 or so agents targeted against Britain during the war, all but one (who committed suicide) had been successfully identified and caught, with several "turned" to become double agents. The system played a major part in the massive campaign of deception which preceded the D-Day landings, designed to give the Germans a false impression of the location and timings of the landings (see Operation Fortitude).
While the double-cross work dealt with enemy agents sent into Britain, a smaller-scale operation run by Victor Rothschild targeted British citizens who wanted to help Germany. The "Fifth Column" operation saw an MI5 officer, Eric Roberts, masquerade as the Gestapo's man in London, encouraging Nazi sympathizers to pass him information about people who would be willing to help Germany in the event of invasion. When his recruits began bringing in intelligence, he promised to pass that on to Berlin. The operation was deeply controversial within MI5, with opponents arguing that it amounted to entrapment. By the end of the war, Roberts had identified around 500 people. But MI5 decided not to prosecute, and instead covered the work up, even giving some of Roberts' recruits Nazi medals. They were never told the truth.
All foreigners entering the country were processed at the London Reception Centre (LRC) at the Royal Patriotic School which was operated by MI5 subsection B1D; 30,000 were inspected at LRC. Captured enemy agents were taken to Camp 020, Latchmere House, for interrogation. This was commanded by Colonel Robin Stephens. There was a Reserve Camp, Camp 020R, at Huntercombe which was used mainly for long term detention of prisoners.
It is believed that two MI5 officers participated in "a gentle interrogation" given to the senior Nazi Heinrich Himmler after his arrest at a military checkpoint in the northern German village of Bremervörde in May 1945. Himmler subsequently killed himself during a medical examination by a British officer by means of a cyanide capsule that he had concealed in his mouth. One of the MI5 officers, Sidney Henry Noakes of the Intelligence Corps, was subsequently given permission to keep Himmler's braces and the forged identity document that had led to his arrest.
The Prime Minister's personal responsibility for the Service was delegated to the Home Secretary Maxwell-Fyfe in 1952, with a directive issued by the Home Secretary setting out the role and objectives of the Director General. The service was subsequently placed on a statutory basis in 1989 with the introduction of the Security Service Act. This was the first government acknowledgement of the existence of the service.
The post-war period was a difficult time for the Service with a significant change in the threat as the Cold War began, being challenged by an extremely active KGB and increasing incidence of the Northern Ireland conflict and international terrorism. Whilst little has yet been released regarding the successes of the service there have been a number of intelligence failures which have created embarrassment for both the service and the government. For instance in 1983 one of its officers, Michael Bettaney, was caught trying to sell information to the KGB. He was subsequently convicted of espionage.
Following the Michael Bettaney case, Philip Woodfield was appointed as a staff counsellor for the security and intelligence services. His role was to be available to be consulted by any member or former member of the security and intelligence services who had "anxieties relating to the work of his or her service" that it had not been possible to allay through the ordinary processes of management-staff relations, including proposals for publications.
The Service was instrumental in breaking up a large Soviet spy ring at the start of the 1970s, with 105 Soviet embassy staff known or suspected to be involved in intelligence activities being expelled from the country in 1971.
Controversy arose when it was alleged that the service was monitoring trade unions and left-wing politicians. A file was kept on Labour Prime Minister Harold Wilson from 1945, when he became an MP, although the agency's official historian, Christopher Andrew maintains that his fears of MI5 conspiracies and bugging were unfounded. As Home Secretary, the Labour MP Jack Straw discovered the existence of his own file dating from his days as a student radical.
One of the most significant and far reaching failures was an inability to conclusively detect and apprehend the "Cambridge Five" spy ring which had formed in the inter-war years and achieved great success in penetrating the government, and the intelligence agencies themselves. Related to this failure were suggestions of a high-level penetration within the service, Peter Wright (especially in his controversial book Spycatcher) and others believing that evidence implicated the former Director General, Roger Hollis or his deputy Graham Mitchell. The Trend inquiry of 1974 found the case unproven of that accusation, and that view was later supported by the former KGB officer Oleg Gordievsky. Another spy ring, the Portland Spy Ring, exposed after a tip-off by Soviet defector Michael Goleniewski, led to an extensive MI5 surveillance operation.
There have been strong accusations leveled against MI5 for having failed in its obligation to provide care for former police agents who had infiltrated the Provisional IRA during the Troubles. The two most notable of the agents, Martin McGartland and Raymond Gilmour, went on to reside in England using false identities and in 2012 launched test cases against the agency. Both men claimed to journalist Liam Clarke in the Belfast Telegraph that they were abandoned by MI5 and were "left high and dry despite severe health problems as a result of their work and lavish promises of life-time care from their former Intelligence bosses". Both men suffer from Post-Traumatic Stress Disorder.
On 12 December 2001, following the United States invasion of Afghanistan, it was agreed that MI5 staff should be sent to Afghanistan to interview prisoners captured by the Northern Alliance and the US military. On 9 January 2002, the first MI5 staff arrived at Bagram. On 12 January 2002, following a report by an MI6 officer that a detainee appeared to have been mistreated before, an MI6 officer was sent instructions that were copied to all MI5 and MI6 staff in Afghanistan about how to deal with concerns over mistreatment, referring to signs of abuse: 'Given that they are not within our custody or control, the law does not require you to intervene to protect this.' It went on to say that the Americans had to understand that the UK did not condone such mistreatment and that a complaint should be made to a senior US official if there was any coercion by the US in conjunction with an MI6 interview.
The Security Service's role in counter-terrorism
The end of the Cold War resulted in a change in emphasis for the operations of the service, assuming responsibility for the investigation of all Irish republican activity within Britain  and increasing the effort countering other forms of terrorism, particularly in more recent years the more widespread threat of Islamic extremism.
Whilst the British security forces in Northern Ireland have provided support in the countering of both republican and loyalist paramilitary groups since the early 1970s, republican sources have often accused these forces of collusion with loyalists. In 2006, an Irish government committee inquiry found that there was widespread collusion between British security forces and loyalist terrorists in the 1970s, which resulted in eighteen deaths. In 2012, a document based review by Sir Desmond de Silva QC into the 1989 murder of Belfast solicitor Patrick Finucane found that MI5 had colluded with the Ulster Defence Association (UDA). The review disclosed that MI5 assessments of UDA intelligence consistently noted that the majority came from MI5 sources with an assessment in 1985 finding 85% came from MI5. Prime Minister David Cameron accepted the findings and apologised on behalf of the British government and acknowledged significant levels of collusion with Loyalists in its state agencies.
On 10 October 2007, the lead responsibility for national security intelligence in Northern Ireland returned to the Security Service from the Police Service of Northern Ireland that had been devolved in 1976 to the Royal Ulster Constabulary (RUC) during Ulsterisation. During April 2010 the Real IRA detonated a 120 lb. car bomb outside Palace Barracks in County Down which is the headquarters of MI5 in Northern Ireland and also home to the 2nd Battalion The Mercian Regiment.
MI5 is understood to have a close working relationship with the Republic of Ireland's Special Detective Unit (SDU), the counter-terrorism and counter-intelligence section of the Garda Síochána (national police), particularly with regards to threats from dissident republican terrorism and Islamic terrorism.
Executive Liaison Groups enable MI5 to safely share secret, sensitive, and often raw intelligence with the police, on which decisions can be made about how best to gather evidence and prosecute suspects in the courts. Each organisation works in partnership throughout the investigation, but MI5 retain the lead for collecting, assessing and exploiting intelligence. The police take lead responsibility for gathering evidence, obtaining arrests and preventing risks to the public.
In 1996, legislation formalised the extension of the Security Service's statutory remit to include supporting the law enforcement agencies in their work against serious crime. Tasking was reactive, acting at the request of law enforcement bodies such as the National Criminal Intelligence Service (NCIS), for whom MI5 agents performed electronic surveillance and eavesdropping duties during Operation Trinity. This role has subsequently been passed to the Serious Organised Crime Agency (SOCA) and then the National Crime Agency (NCA).
In 2001, after the 11 September attacks in the U.S., MI5 started collecting bulk telephone communications data under a little understood general power of the Telecommunications Act 1984 (instead of the Regulation of Investigatory Powers Act 2000 which would have brought independent oversight and regulation). This was kept secret until announced by the Home Secretary in 2015.
In July 2006, parliamentarian Norman Baker accused the British Government of "hoarding information about people who pose no danger to this country", after it emerged that MI5 holds secret files on 272,000 individuals—equivalent to one in 160 adults. It had previously been revealed that a "traffic light" system operates:
- Green: active—about 10% of files
- Amber: enquiries prohibited, further information may be added—about 46% of files.
- Red: enquiries prohibited, substantial information may not be added—about 44% of files
Participation of MI5 agents in criminal activity
In March 2018 the government acknowledged that MI5 agents are allowed to carry out criminal activity in the UK. Mao Foa, the director of Reprieve, said: "After a seven-month legal battle the prime minister has finally been forced to publish her secret order but we are a long way from having transparency. The public and parliament are still being denied the guidance that says when British spies can commit criminal offences and how far they can go. Authorised criminality is the most intrusive power a state can wield. Theresa May must publish this guidance without delay".
In November 2019, four human rights organisations claimed that the UK government has a policy dating from the 1990s to allow MI5 agents to participate in crime and to immunise them against prosecution for criminal actions. The organisations said the policy authorised MI5 agents to participate in criminal activities that protected national security or the economic well-being of the UK. The organisations took the UK government to the Investigatory Powers Tribunal seeking to have it declare the policy illegal and to issue an injunction against further "unlawful conduct". In December 2019 the Tribunal dismissed the request of the human rights organisations in a 3–2 decision. The potential criminal activities include murder, kidnap and torture according to a Bloomberg report.
MI5 was based at Watergate House in the Strand from 1912 until 1916 when it moved to larger facilities at 16 Charles Street for the remaining years of the First World War. After the First World War it relocated to smaller premises at 73-75 Queen's Gate in 1919 and then moved to 35 Cromwell Road in 1929 before transferring to the top floor of the South Block of Thames House on Millbank in 1934. The Service spent the first year of the Second World War at Wormwood Scrubs before moving to Blenheim Palace in 1940. After the Second World War, MI5 was based at Leconfield House (1945–1976) and 140 Gower Street (1976–1994, since demolished) before returning to Thames House in 1994.
The national headquarters at Thames House draws together personnel from a number of locations into a single HQ facility: Thames House also houses the Joint Terrorism Analysis Centre, a subordinate organisation to the Security Service; prior to March 2013, Thames House additionally housed the Northern Ireland Office (NIO). The service has offices across the United Kingdom including an HQ in Northern Ireland.
Directors General of the Security Service
- 1909–1940: Sir Vernon Kell (b. 1873–d. 1942)
- 1940–1941: Oswald Allen Harker (b. 1886–d. 1968)
- 1941–1946: Sir David Petrie (b. 1879–d. 1961)
- 1946–1953: Sir Percy Sillitoe (b. 1888–d. 1962)
- 1953–1956: Dick White (b. 1906–d. 1993)
- 1956–1965: Roger Hollis (b. 1905–d. 1973)
- 1965–1972: Martin Furnival Jones (b. 1912–d. 1997)
- 1972–1979: Michael Hanley (b. 1918–d. 2001)
- 1979–1981: Howard Smith (b. 1919–d. 1996)
- 1981–1985: John Jones (b. 1923–d. 1998)
- 1985–1988: Antony Duff (b. 1920–d. 2000)
- 1988–1992: Patrick Walker (b. 1932)
- 1992–1996: Stella Rimington (b. 1935)
- 1996–2002: Stephen Lander (b. 1947)
- 2002–2007: Eliza Manningham-Buller (b. 1948)
- 2007–2013: Jonathan Evans (b. 1958)
- 2013–2020: Andrew Parker (b. 1962)
- 2020–Present: Ken McCallum
Past names of the Security Service
Although commonly referred to as "MI5", this was the Service's official name for only thirteen years (1916–1929), but it is still used as a sub-title on the various pages of the official Security Service website, as well as in their web address (http://www.mi5.gov.uk).
- October 1909: Founded as the Home Section of the Secret Service Bureau.
- April 1914: Became a subsection of the War Office Directorate of Military Operations, section 5 (MO5)—MO5(g).
- September 1916: Became Military Intelligence section 5—MI5.
- 1929: Renamed the Defence Security Service.
- 1931: Renamed the Security Service.
- Annie Machon – MI5 whistleblower
- David Shayler – MI5 whistleblower
- Club de Berne – a European intelligence sharing forum
- Counter Terrorism Command – of London's Metropolitan Police Service
- Joint Terrorism Analysis Centre
- Spooks – a BBC television drama about the work of a group of MI5 officers (renamed MI-5 in the United States)
- Intelligence and Security Committee of Parliament "Annual Report 2016–2017", page 72. House of Commons (20 December 2017). Retrieved 1 June 2018.
- "Financial Statement 2017-18" (PDF). Security and Intelligence Agencies. p. 13. Retrieved 2 April 2020.
- "What's in a name?". MI5. Retrieved 14 May 2014.
- Geraghty, Tony (2000). The Irish War. London: HarperCollins. ISBN 978-0-00638-674-2.
- "Security Service Act 1989: The Security Service". Legislation.gov.uk. Retrieved 27 August 2017.
- "People and organisation". MI5. Retrieved 21 November 2018.
- "Intelligence Services Act 1994". Legislation.gov.uk. Retrieved 27 August 2017.
- "What the Tribunal can investigate". Investigatory Powers Tribunal. Retrieved 6 July 2014.
- "Freedom of Information Act, section 23". Office of Public Sector Information. Retrieved 3 February 2009.
- Leach, Robert; Coxall, Bill; Robins, Lynton (17 August 2011). British Politics. Palgrave Macmillan. p. 341. ISBN 978-0-230-34422-8. Retrieved 11 July 2015.
- Grierson, Jamie (2 March 2018). "MI5 agents can commit crime in UK, government reveals". The Guardian. Retrieved 3 March 2018.
- "Appointment of the new Director General of the Security Service". Home Office. 28 March 2013. Retrieved 20 August 2013.
- "MI5 – The authorised centenary history". MI5. Archived from the original on 30 September 2013.
- "SIS Records — War Office Military Intelligence (MI) Sections in the First World War". Sis.gov.uk. Archived from the original on 20 August 2006. Retrieved 21 November 2018.
- "End for Special Branch after 122 years". The Telegraph. 9 September 2005. Retrieved 21 November 2018.
- Reginald McKenna, Home Secretary (5 August 1914). "Aliens Restriction Bill". Parliamentary Debates (Hansard). House of Commons. col. 1985.
- Andrew, Christopher (2009). The Defence of the Realm: The Authorised History of MI5. Allen Lane. pp. 49–52.
- Hiley, Nicholas (2006). "Entering the Lists: MI5's great spy round-up of August 1914". Intelligence and National Security. 21 (1): 46–76. doi:10.1080/02684520600568303.
- Andrew, Christopher (2009). The Defence of the Realm: The Authorised History of MI5. Allen Lane. pp. 873–875.
- Hiley, Nicholas (2010). "Re-entering the Lists: MI5's Authorized History and the August 1914 Arrests". Intelligence and National Security. 25 (4): 415–452. doi:10.1080/02684527.2010.537022.
- "How MI5 combated Communist attempts to take over the scouts". The Telegraph. 10 April 2014. Retrieved 21 November 2018.
- "Basil Thomson". Spartacus Educational. Archived from the original on 20 May 2012. Retrieved 1 July 2012.
- "Stephen Searle Hill-Dillon". Bloody Sunday. Retrieved 21 November 2018.
- Hittle, J. B. E. (2011). Michael Collins and the Anglo-Irish War: Britain's Failed Counterinsurgency. Washington, D.C.: Potomac Books. ISBN 978-1-59797-535-3.
- "John Charles Byrnes or Jack Jameson". www.cairogang.com.
- Dwyer, T. Ryle (2005). The Squad and the Intelligence Operations of Michael Collins. Cork: Mercier Press. ISBN 978-1-85635-469-1.
- "Croke Park: Queen in emotionally charged visit". BBC News. 18 May 2011. Retrieved 1 July 2012.
- Kington, Tom (13 October 2009). "Recruited by MI5: the name's Mussolini. Benito Mussolini Documents reveal Italian dictator got start in politics in 1917 with help of £100 weekly wage from MI5". The Guardian. London. Retrieved 14 October 2009.
- "The Cambridge Spies". BBC. Retrieved 1 July 2012.
- Masterman, John C. (1972) . The Double-Cross System in the War of 1939 to 1945. Australian National University Press. ISBN 978-0-7081-0459-0.
- HUTTON, ROBERT. (2019). AGENT JACK : the true story of mi5's secret nazi hunter. [Place of publication not identified]: WEIDENFELD & NICOLSON. ISBN 978-1474605137. OCLC 994362312.
- Hoare, Oliver (2000). Camp 020: MI5 and the Nazi Spies — The Official History of MI5's Wartime Interrogation Centre. Public Record Office. ISBN 978-1-903365-08-3.
- "Heinrich Himmler: How a fake stamp led to the Nazi SS leader's capture". BBC News. 23 May 2020. Retrieved 25 May 2020.
- "Sidney Noakes Intelligence Corps officer with MI5: Himmler's false identity document". Military Intelligence Museum. Retrieved 25 May 2020.
- "Security Service Act 1989". 4 July 2000. Retrieved 1 July 2012.
- Harrison, David (11 November 2007). "Cold War rivals play at spy game". The Daily Telegraph. Retrieved 1 July 2012.
- The Earl of Caithness, Minister of State, Home Office (30 November 1987). "Security Services Ombudsman: Access". Parliamentary Debates (Hansard). House of Lords. col. 811.
- John Patten, Minister for Home Affairs (21 December 1988). "Official Secrets Bill". Parliamentary Debates (Hansard). House of Commons. col. 538.
- Hollingsworth, Mark; Norton-Taylor, Richard (1988). Blacklist: The Inside Story of Political Vetting. London: Hogarth Press. p. 104. ISBN 978-0-70120-811-0.
- "MI5 kept file on former PM Wilson". BBC News. 3 October 2009.
- Schaefer, Sarah (22 January 1999). "Parliament & Politics: Straw will not see his MI5 file". The Independent. Retrieved 1 July 2012.
- Bamford, James (18 November 1990). "Gordievsky's People". The New York Times. Retrieved 1 July 2012.
- Lewis, Jason; Wynne-Jones, Jonathan (18 June 2011). "MI5 labelled the Archbishop of Canterbury a subversive over anti-Thatcher campaigns". The Daily Telegraph. Retrieved 1 July 2012.
- Clarke, Liam (14 September 2012). "Two ex-spies target MI6 in landmark legal battle over payouts". Belfast Telegraph. Retrieved 7 January 2013.
- Corera, Gordon, MI6: Life and Death in the British Secret Service, W&N , 2012, ISBN 0753828332, 978-0753828335, p.339
- "7 Tied to Faction of the I.R.A. Face Terrorism Charges". The New York Times. 19 May 2012. Retrieved 20 November 2011.
- Palmer, Alasdair (14 May 2006). "MI5 mission: impossible". The Daily Telegraph. Retrieved 1 July 2012.
- "Barron finds British collusion in attacks". The Irish Times. 29 November 2006.
- "Final Report on the Report of the Independent Commission of Inquiry into the Bombing of Kay's Tavern, Dundalk" (PDF). Houses of the Oireachtas. November 2006. Archived from the original (PDF) on 26 October 2008. Retrieved 20 November 2011 – via Burnsmoley.com.
- "Volume 1 Chapter 11: The flow of information from members of the security forces to the UDA". Pat Finucane Review. Archived from the original on 16 December 2012.
- "Pat Finucane murder: 'Shocking state collusion', says PM". BBC News. 12 December 2012. Retrieved 16 July 2017.
- "MI5 in Northern Ireland". Security Service MI5. Retrieved 15 July 2017.
- "Transfer of national security lead to the Security Service". Police Service of Northern Ireland. Archived from the original on 8 June 2008.
- "Man arrested over Palace Barracks bomb released". BBC News. 9 May 2010. Retrieved 1 July 2012.
- McDonald, Henry (2 March 2008). "MI5 targets Ireland's al-Qaeda cells". The Guardian. Retrieved 5 June 2014.
- Howells, Kim (May 2009). Could 7/7 Have Been Prevented? Review of the Intelligence on the London Terrorist Attacks on 7 July 2005 (PDF). London: UK Cabinet Office, Intelligence and Security Committee. Archived from the original (PDF) on 6 June 2009.
- Baroness Blatch, Minister of State, Home Office (10 June 1996). "Security Service Bill". Parliamentary Debates (Hansard). House of Commons. col. 1502–1503.
- "About us". National Crime Agency. Retrieved 21 November 2018.
- Corera, Gordon (5 November 2015). "How and why MI5 kept phone data spy programme secret". BBC News. Retrieved 9 November 2015.
- Whitehead, Tom (4 November 2015). "MI5 and GCHQ secretly bulk collecting British public's phone and email records for years, Theresa May reveals". The Daily Telegraph. Retrieved 9 November 2015.
- "Here's the little-known legal loophole that permitted mass surveillance in the UK". The Register. 9 November 2015. Retrieved 9 November 2015.
- Jack Straw, Home Secretary (25 February 1998). "Security Service Files". Parliamentary Debates (Hansard). House of Commons. col. 346–348.
- "MI5 Files". Parliamentary Debates (Hansard). House of Commons. 5 June 2006. col. 278W.
- Osborne, Samuel (8 November 2019). "MI5 licensed informants to commit murder, kidnap and torture for decades, court hears". The Independent. Retrieved 9 November 2019.
- Browning, Jonathan (21 December 2019). "Court Rules British MI5 Agents Can Murder, Kidnap and Torture". Bloomberg. Retrieved 2 January 2020.
- Andrew, Christopher (2009). The Defence of the Realm: The Authorized History of MI5. Allen Lane. p. 85. ISBN 978-0-713-99885-6.
- Andrew, Christopher (2009). The Defence of the Realm: The Authorized History of MI5. Allen Lane. p. 117. ISBN 978-0-713-99885-6.
- Andrew, Christopher (2009). The Defence of the Realm: The Authorized History of MI5. Allen Lane. p. 134. ISBN 978-0-713-99885-6.
- Andrew, Christopher (2009). The Defence of the Realm: The Authorized History of MI5. Allen Lane. p. 217. ISBN 978-0-713-99885-6.
- "MI5 (The Security Service)". The Secret Architecture of London. Retrieved 18 February 2017.
- Sheldon, Robert (June 1993). Thames House and Vauxhall Cross (PDF). London: National Audit Office. p. 43. ISBN 978-0-10556-669-4. Retrieved 7 July 2013.
- "Intelligence, Counter-terrorism and Trust". MI5 (Press release). 5 November 2007. Retrieved 8 January 2015.
- Leppard, David (14 June 2009). "Oops! Building firm blurts out secrets of hush-hush MI5 HQ". The Sunday Times. Archived from the original on 15 June 2010. Retrieved 28 December 2011.