The UK's intelligence network consists of five agencies of which the main three - MI5, MI6 and GCHQ - command a combined budget of just under £900m a year. Between them they run agents, combat internal subversion and identify threats to the UK from abroad.
SECRET INTELLIGENCE SERVICE, MI6
MI6 had its origins in the Secret Service Bureau before World War I, as did MI5.
Its first leader was Sir Mansfield Cumming, who always signed himself "C", a tradition carried on by his successors.
The department falls under the remit of the Foreign Office, and is based in an eye-catching building in Vauxhall in central London.
It was MI6 which supplied much of the information that went into the September 2002 intelligence dossier on Iraq, notably the single-sourced second hand reports that Iraq was able to launch weapons of mass destruction within 45 minutes of an order being given.
Government Communications Headquarters, set up in 1946, was born out of the wartime code-breakers based in Bletchley Park.
Based in Cheltenham, GCHQ has two functions: To monitor, intercept, and decrypt information from those who pose a threat to the UK, and to keep confidential government information secret.
Staffed by civil servants, including skilled mathematicians and linguists, GCHQ comes under the responsibility of the Foreign Office.
SECURITY SERVICE, MI5
One of the UK's three security services, alongside the Secret Intelligence Service and GCHQ.
MI5's task is to maintain Britain's internal security, and as such it falls under the jurisdiction of the Home Office.
The forerunner of MI5, the Secret Service Bureau, was born in 1909 in order to protect Britain from its then principal enemy, Germany. In 1916 it became known as Military Intelligence.
Over the decades MI5 has sought to protect the UK during two world wars and the cold war and from internal communist and fascist subversion.
From the 1990s, and especially following the 11 September 2001 attacks in the US, MI5 has spent more and more of its resources targeting terrorism.
DEFENCE INTELLIGENCE STAFF
The DIS is part of the Ministry of Defence, and has existed in its present form since the 1960s.
It has about 4,500 staff, whose primary function is to provide information on possible threats to the UK, and to assess intelligence material.
More recently the DIS's profile was raised during the Hutton Inquiry.
Brian Jones, now retired, was a senior member of the DIS who has raised concerns that the 2002 Iraq dossier removed essential caveats when describing Iraq's chemical and biological weapons.
JOINT INTELLIGENCE COMMITTEE
The role of JIC is to advise ministers on priorities for intelligence gathering, as well as analysing information produced by MI5, MI6, the Defence Intelligence staff and GCHQ. The JIC is part of the Cabinet Office.
The committee's chief, John Scarlett, was in charge of producing the September 2002 Iraq dossier.
During the Hutton inquiry he strongly denied claims that Downing Street had insisted on including intelligence in the dossier on Iraq's ability to launch weapons of mass destruction within 45 minutes.
Mr Scarlett has since been appointed as the new head of MI6.