MI5

The Security Service comes under the authority of the Home Secretary within the Cabinet. The service is headed by a Director General (DG) at the grade of a Permanent Secretary of the 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 current Director General is Ken McCallum, who succeeded Andrew Parker in April 2020.

The service is directed for intelligence operational priorities by the Joint Intelligence Committee.

Operations of the service are required to be proportionate, and compliant with British legislation, including the Regulation of Investigatory Powers Act 2000, the Investigatory Powers Act 2016, the Data Protection Act 2018, and various other items of legislation. Information held by the service is exempt from disclosure under section 23 of the Freedom of Information Act 2000. The Security Service Act of 1989 establishes several legal key mechanisms aimed at ensuring the accountability and control of the Service. Certain provisions from this act were later integrated into the Intelligence Services Act of 1994 with European Commission on Human Rights later having endorsed these mechanisms in multiple applications under the European Convention on Human Rights. Additionally, all employees of the service are bound by the Official Secrets Act 1989. The have a duty work with the police to investigate any criminal offences under the National Security Act 2023 and The Official Secrets Act 1989

The Security Service "is authorised to investigate any person or movement that might threaten the ... security" of the United Kingdom.

The Service is subject to oversight and accountability through various means. …

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. The Bureau was initially split into naval and army sections which, over time, specialised respectively in foreign target espionage and internal counter-espionage activities. The former specialisation was a result of a growing interest at the Admiralty, at the time, in intelligence regarding the fleet of the Imperial German Navy. This division was formalised, as separate home and foreign sections, prior to the beginning of the First World War. Following a number of administrative changes, the home section became known as Directorate of Military Intelligence, Section 5 and the abbreviation MI5, the name by which it is still known in popular culture. (The foreign/naval section of the Secret Service Bureau was to become the basis of the later Secret Intelligence Service, or MI6).

The founding head of the Army section was Vernon Kell of the South Staffordshire Regiment, who remained in that role until the early part of the Second World War. Its role was originally quite restricted, as the section existed solely to ensure national security through counter-espionage. With a small staff, and working in conjunction with the Special Branch of the Metropolitan Police, the service was responsible for overall direction and the identification of foreign agents, while Special Branch provided the manpower for the investigation of their affairs, arrest and interrogation.

On the day after the declaration of the First World War, the Home Secretary, Reginald McKenna, 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". These arrests have provoked recent historical controversy. According to the official history of MI5, the actual number of agents identified was 22, and Kell had started sending out letters to local police forces on 29 July, giving them advance warning of arrests to be made as soon as war was declared. …

MI5 was based at Watergate House in the Strand from 1912 until 1916, when it moved to larger facilities at 16 Charles Street for the remaining years of the First World War. After the First World War, it relocated to smaller premises at 73–75 Queen's Gate in 1919, and then moved to 35 Cromwell Road in 1929, before transferring to the top floor of the South Block of Thames House on Millbank in 1934. The Service spent the first year of the Second World War at Wormwood Scrubs, before moving to Blenheim Palace, Oxfordshire, in 1940. After the Second World War, MI5 was based at Leconfield House (1945–1976), and 140 Gower Street (1976–1994, since demolished), before returning to Thames House in 1994.

The national headquarters at Thames House draws together personnel from a number of locations into a single HQ facility: Thames House also houses the Joint Terrorism Analysis Centre (JTAC), a subordinate organisation to the Security Service; prior to March 2013, Thames House additionally housed the Northern Ireland Office (NIO). The service has offices across the United Kingdom, including a HQ in Northern Ireland. Details of a northern operations centre in Greater Manchester were revealed by the firm who built it.

Although the Service is commonly referred to as 'MI5', this was its official name for only thirteen years (1916–1929). However, it is still used as a sub-title on the various pages of the official Security Service website, as well as in their web address (https://www.MI5.gov.uk).

MI5 has previously used Government Communications Planning Directorate (GCPD) as a cover name when sponsoring research related to drone usage in charging recording devices.