Who is considered the biggest mole in British Intelligence?

Harold Adrian Russell "Kim" Philby (1 January 1912 – 11 May 1988) was a high-ranking member of British intelligence who worked as a double agent before defecting to the Soviet Union in 1963. He served as both an NKVD and KGB operative.

In 1963, Philby was revealed to be a member of the spy ring now known as the Cambridge Five, the other members of which were Donald Maclean, Guy Burgess, Anthony Blunt and, possibly, John Cairncross. Of the five, Philby is believed to have been most successful in providing secret information to the Soviet Union. His activities were moderated only by Joseph Stalin's fears that he was secretly on Britain's side. Philby was an Officer of the Order of the British Empire (OBE) from 1946 to 1965.

In a 1981 lecture to the East German security service, Stasi, Philby attributed the failure of the British Secret Service to unmask him as due in great part to the British class system—it was inconceivable that one "born into the ruling class of the British Empire" would be a traitor—to the amateurish and incompetent nature of the organisation, and to so many in MI6 having so much to lose if he was proven to be a spy. He had the policy of never confessing—a document in his own handwriting was dismissed as a forgery.