Harold Adrian Russell 'Kim' Philby (1912-1988) was a senior spy working for British Intelligence (SIS) -- or so they thought...
He was, in actual fact, a KGB agent. As head of SIS's anti-Soviet section, he was able to betray large numbers of British agents to the Russians. Philby also served as a chief British intelligence officer in America. He was unmasked in 1963, when he fled to the Soviet Union. He lived the rest of his life in Moscow, dying shortly before the collapse of the USSR.
Like LMS, Philby had an earlier career as a journalist, and was a war correspondent in Spain. He joined SIS in 1940.
Kim Philby was a member of the ring of spy traitors later known as the Cambridge Five.