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The Security Service, also known as 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. The service is directed to protect British parliamentary democracy and economic interests, and counter terrorism and espionage within the UK.
The Security Service comes under the authority of the Home Secretary within the Cabinet. The service is headed by a Director General at the grade of a Permanent Secretary of the British Civil Service who is directly supported by an internal security organisation, secretariat, legal advisory branch and information services branch. The Deputy Director General is responsible for the operational activity of the service, being responsible for four branches; international counter-terrorism, National Security Advice Centre (counter proliferation and counter espionage), Irish and domestic counter-terrorism and technical and surveillance operations.
The service is directed by the Joint Intelligence Committee for intelligence operational priorities. It liaises with SIS, GCHQ, DIS, and a number of other bodies within the British government and industrial base. It is overseen by the Intelligence and Security Committee of Members of Parliament, who are directly appointed by the Prime Minister, by the Interception of Communications Commissioner, and by the Intelligence Services Commissioner. Judicial oversight of the service's conduct is exercised by the Investigatory Powers Tribunal.