Although Britain’s various intelligence organizations can be traced back to the Elizabethan era, the modern security and intelligence structure is based on the separation of responsibilities established during World War I when the Home and Foreign departments of the Secret Service Bureau evolved into the Security Service (MI5) and the Secret Intelligence Service (SIS), leaving the Admiralty with the supervision of signals interception and decryption. In 1919 signals intelligence was passed to the Government Code and Cypher School (GC&CS) and continued under SIS’s umbrella until 1946 when, as the Government Communications Headquarters (GCHQ), it moved from Bletchley Park to Eastcote and was placed under the control of the Foreign Office. Accordingly, the United Kingdom now maintains three principal intelligence agencies—MI5, SIS, and GCHQ—with a total budget in excess of a billion pounds. Now based at Benhall in Cheltenham, GCHQ employs more staff than the other two agencies combined, although exact strengths are not published.
During World War II numerous other agencies enjoyed a brief existence, including the Special Operations Executive (SOE), which conducted resistance and sabotage operations in Nazi-occupied territory, and British Security Coordination, which acted as an umbrella for MI5, SIS, SOE, and the Political Warfare Executive in New York.
None survived significantly into the postwar era, although regional representative organizations were maintained in the Middle East (Security Intelligence Middle East and the Inter-Services Liaison Department), Far East (Combined Intelligence Far East), Iraq (Combined Intelligence Centre Iraq), and the Aden Intelligence Centre.