MI5 vs MI6: The Difference in British Intelligence

The difference between MI5 and MI6 comes down to a border. MI5, the Security Service, protects the United Kingdom from within through counter-terrorism and counter-espionage. MI6, the Secret Intelligence Service (SIS), gathers human intelligence abroad. Both grew out of the same office in 1909 and, together with GCHQ, they form the core of Britain's intelligence community.

At a glance: MI5 compared with MI6

One bureau, two services

Worried about German espionage, the British government set up the Secret Service Bureau in 1909. It soon split into a home section under Captain Vernon Kell and a foreign section under Commander Mansfield Cumming. Those halves became MI5 and MI6, labels taken from the numbering of military intelligence sections during the First World War. Cumming signed his papers 'C', and SIS chiefs still use the initial.

What MI5 actually does

MI5 investigates threats to national security inside the UK: terrorism, hostile state espionage and the spread of weapons of mass destruction. Since 2007 it has also led on intelligence about Northern Ireland-related terrorism. Its officers cannot arrest anyone, so when a case reaches that point MI5 works with the police, above all counter-terrorism policing.

Its best-known wartime success was the Double Cross System. German agents caught or turned in Britain fed carefully controlled information back to their handlers, and the scheme played a part in the deception that preceded D-Day.

What MI6 actually does

SIS recruits and runs human sources outside the UK to obtain secret information about foreign governments, armed groups and other threats, the discipline known as HUMINT. The government did not officially acknowledge that SIS existed until 1992, and the service was given a legal footing by the Intelligence Services Act 1994.

Where GCHQ fits in

GCHQ is the third major agency, responsible for signals intelligence (SIGINT) and cyber security. It descends from the codebreakers of Bletchley Park. The three agencies share material, and their assessments feed the Joint Intelligence Committee in the Cabinet Office.

Key dates

• 1909: Secret Service Bureau founded, then split into home and foreign branches.
• 1939-1945: MI5 runs the Double Cross System.
• 1951-1963: the Cambridge Five spy ring, including Kim Philby inside SIS, becomes Britain's worst counter-intelligence failure.
• 1989: Security Service Act puts MI5 on a statutory basis.
• 1992: Stella Rimington becomes the first Director General to be publicly named; SIS is officially acknowledged.
• 1994: Intelligence Services Act covers SIS and GCHQ and creates the parliamentary Intelligence and Security Committee.
• 2016: Investigatory Powers Act sets out interception powers and safeguards.

Who watches the watchers

All three agencies answer to Parliament's Intelligence and Security Committee, whose powers were strengthened by the Justice and Security Act 2013, and to the Investigatory Powers Commissioner. Members of the public can complain to the Investigatory Powers Tribunal. Lawyers and civil liberties groups still disagree about whether this oversight goes far enough.