Ring of Five
The Ring of Five, also known as the Cambridge Five, were a group of British citizens recruited as Communist spies before WWII, while they were at the University of Cambridge:
Three of the five were discovered relatively soon after the war. Burgess and Maclean, who worked in the Foreign Office, defected in 1951. Philby, who had risen to head the Soviet counterintelligence division in the Secret Intelligence Service (SIS), was removed from office in 1951 — as the chief officer assigned to counter Soviet intelligence.
Blunt, who had worked in the Security Service, was secretly identified in 1964 but announced in 1979. There was much speculation about the "Fifth Man", but the involvement of Cairncross, a SIS ofificer was announced in 1990.