The Security Service, also MI5 (Military Intelligence, Section 5), is the United Kingdom's domestic counter-intelligence and security agency and is part of its intelligence machinery alongside the Secret Intelligence Service (MI6), Government Communications Headquarters (GCHQ) and Defence Intelligence (DI). MI5 is directed by the Joint Intelligence Committee (JIC), and the service is bound by the Security Service Act 1989 and the Intelligence Services Act 1994. The service is directed to protect British parliamentary democracy and economic interests, and counter terrorism and espionage within the UK.
Within the civil service community the service is colloquially known as Box 500 (after its official wartime address of PO Box 500; its current address is PO Box 3255, London SW1P 1AE).
The service has had a national headquarters at Thames House on Millbank in London since 1995, drawing together personnel from a number of locations into a single HQ facility.
Thames House was, until March 2013, shared with the Northern Ireland Office (NIO) and is also home to the Joint Terrorism Analysis Centre, a subordinate organisation to the Security Service. The service has offices across the United Kingdom including an HQ in Northern Ireland.
Details of the northern operations centre in Greater Manchester were revealed by the firm who built it. Plans to open the northern operations centre were reported by The Manchester Evening News in February 2005, and plans to open a permanent Scottish office in Glasgow were reported by The Scotsman in January of that year.
Britain's internal security service, MI5, has held the first photocall in its 84-year history.
Stella Rimington, 56, became the first Director General of the MI5 to pose openly for cameras at the launch of a brochure outlining the organisation's activities
She was made the first female head of MI5 last year - following another unprecedented public announcement - after 22 years in the service.
Journalists were invited to their first official briefing by a senior MI5 official where they were warned that this was not the beginning of "a great avalanche of openness".
The difference between MI5 and MI6
The MI5 seeks to protect the United Kingdom, its citizens and interests from internal and external threats while the MI6 focuses exclusively on external threats. The MI6 is officially known as the Secret Intelligence Service.
The MI6 is the government agency to which the famous international spy, James Bond, belongs.