# MI5 The Security Service, commonly known as MI5, is the United Kingdom's domestic counterintelligence and security agency — responsible for protecting the UK against threats from espionage, terrorism, and sabotage within British territory. It is the British equivalent of the [[FBI]]'s national security division and the counterpart to [[MI6]] (foreign intelligence) and [[GCHQ]] (signals intelligence). MI5 operates under the authority of the Home Secretary. Unlike [[MI6]] and [[GCHQ]], which operate abroad, MI5 focuses on threats inside the UK: hostile foreign intelligence operations on British soil, Islamist and far-right terrorism, Northern Ireland-related security, and the protection of critical national infrastructure. It has no arrest powers — when MI5 identifies a suspect for prosecution, it hands the case to police. ## Historical significance MI5's history includes running the Double Cross system during World War II (turning every German agent in Britain into a double agent feeding disinformation back to the Abwehr), the decades-long hunt for Soviet moles that consumed the agency during the Cold War, and the transformation into a counterterrorism-focused organization after 9/11 and the 7/7 London bombings (2005). The Peter Wright affair — in which a former MI5 officer alleged in his memoir "Spycatcher" (1987) that MI5 director-general Roger Hollis had been a Soviet agent — exemplified the institutional paranoia that the Cambridge Five penetration created. The British government's attempt to suppress the book globally became a landmark censorship case. ## KB relevance MI5 connects to the vault through the [[Five Eyes Alliance]] domestic intelligence-sharing arrangements, the [[Surveillance State]] debate (MI5's bulk data collection powers under the Investigatory Powers Act 2016 — dubbed the "Snoopers' Charter"), and as a comparison point for American debates about [[FBI]] domestic surveillance authorities under [[FISA Section 702]]. ## See Also - [[MI6]] — UK foreign intelligence - [[GCHQ]] — UK signals intelligence - [[FBI]] — US domestic counterintelligence equivalent - [[Five Eyes Alliance]] — the intelligence-sharing framework - [[Surveillance State]] — the domestic monitoring apparatus - [[Global Intelligence Hub]] — full agency index