It liaises with SIS, GCHQ, DIS, and a number of other bodies within the British government and industrial base.

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On the day after the declaration of World War I, the Home Secretary, Reginald Mc Kenna, 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.

Information held by the service is exempt from disclosure under section 23 of the Freedom of Information Act 2000.

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.

MI5 acquired many additional responsibilities during the war.

Most significantly, its strict counter-espionage role blurred considerably.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.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.It was consistently successful throughout the rest of the 1910s and 1920s in its core counter-espionage role.