The government agency in the United Kingdom that is responsible for internal security and counterintelligence on British territory WordNet 3.0 & Open English WordNet

The Security Service, also known as MI5, is the United Kingdom's domestic counter-intelligence and security agency and is part of its intelligence machinery alongside the Secret Intelligence Service, Government Communications Headquarters, and Defence Intelligence. Wikipedia

MI6, MI7, MI8, or MI9, the United Kingdom military intelligence sections Wikipedia Disambiguation

The domestic security service of the United Kingdom Wikipedia Disambiguation

The domestic intelligence and counter-espionage agency of the United Kingdom, which operates in secret and whose primary function is to spy on terrorist organizations and others viewed as a threat to British national security. Formally known as the Security Service. Wiktionary