TheGovernment of the United Kingdom maintains severalintelligence agencies that deal with secret intelligence. These agencies are responsible forcollecting,analysing and exploiting foreign and domestic intelligence, providingmilitary intelligence, and performingespionage and counter-espionage. Their intelligence assessments contribute to the conduct of theforeign relations of the United Kingdom, maintaining thenational security of the United Kingdom,military planning,public safety, andlaw enforcement in the United Kingdom. The four main agencies are theSecret Intelligence Service (SIS or MI6), theSecurity Service (MI5), theGovernment Communications Headquarters (GCHQ) andDefence Intelligence (DI). The agencies are organised under threegovernment departments, theForeign Office, theHome Office and theMinistry of Defence.

Although the history of the organisations dates back to the 19th century or earlier, the British intelligence systems as we know it today--with components for domestic, foreign, military, and communications intelligence--did not emerge until the years immediately preceding World War I.^[1] The decryption of theZimmermann Telegram in 1917 was described as the most significant intelligence triumph for Britain during World War I,^[2] and one of the earliest occasions on which a piece of signals intelligence influenced world events.^[3] During the Second World War and afterwards, many observers regardedUltra signals intelligence as immensely valuable to theAllies of World War II. In 1962, during theCuban Missile Crisis, GCHQ interceptions of Soviet ship positions were sent directly to theWhite House.^[4] Intelligence cooperation in the post-war period between the United Kingdom and the United States became the cornerstone of Western intelligence gathering and the "Special Relationship" between the United Kingdom and the United States.^[5]

The UK Intelligence Community consists of the following.^[6]

• National Security Council
• Joint Intelligence Committee

TheNational Security Adviser (NSA) is a senior official in theCabinet Office, based inWhitehall, who serves as the principal adviser to thePrime Minister of the United Kingdom andCabinet of the United Kingdom on allnational security issues.^[7]

TheIntelligence and Security Committee of Parliament oversees the policies, expenditure, administration and operations of the UK Intelligence Community.^[6] Complaints are heard by theInvestigatory Powers Tribunal. Other oversight is provided by theInvestigatory Powers Commissioner and theIndependent Reviewer of Terrorism Legislation.

The following agencies, groups and organisations fall under the remit of the Intelligence and Security Committee of Parliament:^[6]

Four national centres and authorities support and complement the work of the agencies:

• HM Government Communications Centre (HMGCC), provides electronics and software to support the communication needs of theUK Government;
• National Cyber Security Centre (NCSC), a child agency of GCHQ
• National Protective Security Authority (NPSA), a child agency of MI5;
• UK National Authority for Counter Eavesdropping (UK NACE), part ofFCDO Services.

Organised intelligence collection and planning for theGovernment of the United Kingdom and theBritish Empire was established during the 19th century. TheWar Office, responsible for the administration of theBritish Army, formed the Intelligence Branch in 1873, which became theDirectorate of Military Intelligence. TheAdmiralty, responsible for command of theRoyal Navy, formed the Foreign Intelligence Committee in 1882,^[15] which evolved into theNaval Intelligence Department (NID) in 1887.^[16]

TheCommittee of Imperial Defence, established in 1902, was responsible for research, and some co-ordination, on issues ofmilitary strategy.

The Secret Service Bureau was founded in 1909 as a joint initiative of the Admiralty and the War Office to control secret intelligence operations in the UK and overseas, particularly concentrating on the activities of theImperial German government. The Bureau was split into naval and army sections which, over time, specialised in foreign espionage and internal counter-espionage activities respectively. This specialisation, formalised before 1914, was a result of the Admiralty intelligence requirements related to the maritime strength of the Imperial German Navy.

In 1916, during theFirst World War, the two sections underwent administrative changes so that the internal counter-espionage section became the Directorate of Military Intelligence Section 5 (MI5) and the foreign section became the Directorate of Military Intelligence Section 6 (MI6), names by which the Security Service and Secret Intelligence Service are commonly known today.

TheNaval Intelligence Division led the Royal Navy's highly successful cryptographic efforts,Room 40 (later known as NID25). The decryption of theZimmermann Telegram was described as the most significant intelligence triumph for Britain during World War I,^[2] and one of the earliest occasions on which a piece of signals intelligence influenced world events.^[3]

TheImperial War Cabinet was the British Empire's wartime coordinating body.

In 1919, the Cabinet's Secret Service Committee recommended that a peacetime codebreaking agency should be created.^[17] Staff were merged from NID25 andMI1b into the new organisation,^[18] which was given the cover-name the "Government Code and Cypher School" (GC&CS).^[19]

TheJoint Intelligence Committee (JIC) was founded in 1936 as a sub-committee of the Committee of Imperial Defence.^[20]

Following the outbreak of theSecond World War in 1939, the JIC became the senior intelligence assessment body for the United Kingdom government. During the War, theRAF Intelligence Branch was established, although personnel had been employed in intelligence duties in the RAF since its formation in 1918. TheSpecial Operations Executive (SOE) was operational from 1940 until early 1946. SOE conductedespionage,sabotage andreconnaissance inoccupied Europe and later inoccupied Southeast Asia against theAxis powers and aided localresistance movements.

The 1943 British–US Communication Intelligence Agreement,BRUSA, connected the signal intercept networks of the GC&CS and the U.S.National Security Agency.^[21] The GC&CS was based largely atBletchley Park. Its staff, includingAlan Turing, worked oncryptanalysis of the Enigma (codenamedUltra) andLorenz cipher,^[22] and also a large number of other enemy systems.Winston Churchill was reported to have toldKing George VI, when presenting to himStewart Menzies (head of theSecret Intelligence Service and the person who controlled distribution of Ultra decrypts to the government): "It is thanks to the secret weapon of General Menzies, put into use on all the fronts, that we won the war!"^[23]F. W. Winterbotham quoted the western Supreme Allied Commander,Dwight D. Eisenhower, at war's end describing Ultra as having been "decisive" to Allied victory.^[24]Sir Harry Hinsley, Bletchley Park veteran and official historian of British Intelligence in World War II, made a similar assessment about Ultra, saying that it shortened the war "by not less than two years and probably by four years"; and that, in the absence of Ultra, it is uncertain how the war would have ended.^[25]

The Government Code and Cypher School was renamed the "Government Communications Headquarters" (GCHQ) in 1946.^[26] The Joint Intelligence Bureau (JIB) was established the same year.^[27] It was structured into a series of divisions: procurement (JIB 1), geographic (JIB 2 and JIB 3), defences, ports and beaches (JIB 4), airfields (JIB 5), key points (JIB 6), oil (JIB 7) and telecommunications (JIB 8).^[28]

Wartime signals intelligence cooperation between the United Kingdom and the United States continued in the post-war period.^[29] The two countries signed the bilateralUKUSA Agreement in 1948.^[30] Later broadened to include Canada, Australia and New Zealand, known as theFive Eyes, as well as cooperation with several "third-party" nations, this became the cornerstone of Western intelligence gathering and the "Special Relationship" between the UK and the USA.^[5] Since World War II, the chief of the London station of the U.S.Central Intelligence Agency has attended the Joint Intelligence Committee's weekly meetings. One former US intelligence officer has described this as the "highlight of the job" for the London CIA chief.^[31] Resident intelligence chiefs fromAustralia,Canada, andNew Zealand may attend when certain issues are discussed.^{[citation needed]}

The Joint Intelligence Committee moved to theCabinet Office in 1957 with its assessments staff who prepared intelligence assessments for the committee to consider.

During theCuban Missile Crisis,GCHQ Scarborough intercepted radio communications from Soviet ships reporting their positions and used that to establish where they were heading. A copy of the report was sent directly to the White House Situation Room, providing initial indications of Soviet intentions with regard to the US naval blockade of Cuba.^[4]

When theMinistry of Defence was formed in 1964, the Joint Intelligence Bureau, Naval Intelligence, Military Intelligence and Air Intelligence were combined to form theDefence Intelligence Staff (DIS).^[32] The DIS focussed initially on Cold War issues.^[33]

As well as a mission to gather intelligence, GCHQ has for a long time had a corresponding mission to assist in the protection of the British government's communications. Building on the work ofJames H. Ellis in the late 1960s,Clifford Cocks invented apublic-key cryptography algorithm in 1973 (equivalent to what would become, in 1978, theRSA algorithm), which was shared with the National Security Agency in the United States.^[34]

TheSecurity Service Act 1989 established the legal basis of the Security Service (MI5) for the first time under thegovernment led by Margaret Thatcher. GCHQ and the Secret Intelligence Service (MI6) were placed on a statutory footing by theIntelligence Services Act 1994 under thegovernment led by John Major.

TheNational Infrastructure Security Coordination Centre (NISCC) and theNational Security Advice Centre (NSAC) were formed in 1999. NISCC's role was to provide advice to companies operatingcritical national infrastructure,^[35] and NSAC was a unit within MI5 that provided security advice to other parts of the UK government. TheCentre for the Protection of National Infrastructure (CPNI) was formed as a child agency of MI5 in 2007, merging the NISCC and NSAC.^[36] CPNI provided integrated security advice (combining information, personnel, and physical) to the businesses and organisations which made up the critical national infrastructure.^[37] In 2016, the cybersecurity-related aspects of the CPNI's role were taken over by the newly-formedNational Cyber Security Centre (NCSC), a child agency of GCHQ.^[38] The CPNI evolved into theNational Protective Security Authority (NPSA) in 2023, taking on a remit beyond critical national infrastructure.^[39]

TheOffice for Security and Counter-Terrorism (OCST) was created in 2007 and is responsible for leading work oncounter-terrorism working closely with thepolice and security services. The OSCT was renamed the Homeland Security Group in 2021.

The Defence Intelligence Staff changed its name toDefence Intelligence (DI) in 2009.^[33] Defence Intelligence has a unique position within the UK intelligence community as an 'all-source' intelligence function.

TheNational Security Council (NSC) was established in 2010, reestablishing the central coordination of national security issues seen in the Committee of Imperial Defence.^[40] TheJoint Intelligence Organisation was formalised to provide intelligence assessment and advice on development of the UK intelligence community's analytical capability for the Joint Intelligence Committee and NSC.^[41]

The heads of the Five Eyes domestic security agencies gave a public presentation together for the first time in 2023.^[42] TheMI6 chief and theCIA director made their first-ever joint remarks in anopinion piece in theFinancial Times in 2024.^[43][44]

TheSingle Intelligence Account (SIA) is the funding vehicle for the three main security and intelligence agencies: theSecret Intelligence Service (SIS/MI6),^[45]Government Communications Headquarters (GCHQ)^[46] and theSecurity Service (MI5).^[47] Spending on the SIA was £3.6 billion infinancial year 2022/23.^[48]

Defence Intelligence is an integral part of theMinistry of Defence (MoD) and is funded within the UK's defence budget.

• Counter Terrorism Command
• Club de Berne, an intelligence-sharing forum that includes the UK
• Five Eyes, an intelligence-sharing alliance that includes the UK
• Information Research Department
• Intelligence Corps (United Kingdom)
• List of intelligence agencies global list sorted by country
• Mass surveillance in the United Kingdom
• UK cyber security community

• Dylan, Huw (2014).Defence Intelligence and the Cold War: Britain's Joint Intelligence Bureau 1945-1964. Oxford University Press.ISBN 978-0199657025.
• Hinsley, Sir Harry (1996) [1993],The Influence of ULTRA in the Second World War(PDF), retrieved23 July 2012 Transcript of a lecture given on Tuesday 19 October 1993 at Cambridge University
• Johnson, John (1997).The Evolution of British Sigint: 1653–1939. HMSO.ASIN B002ALSXTC.
• Winterbotham, F. W. (1974),The Ultra Secret, New York: Harper & Row,ISBN 978-0-06-014678-8 The first published account of the previously secret wartime operation, concentrating mainly on the distribution of intelligence. It was written from memory and has been shown by subsequent authors, who had access to official records, to contain some inaccuracies.

• The Secret History of GCHQ BBC documentary