Kim Philby

Harold Adrian Russell "Kim" Philby (1 January 1912 – 11 May 1988) was a British intelligence officer who worked as a spy for the Soviet Union, before defecting in 1963. 
Philby began his work for the Soviet Union as a spy in 1934. He went on to serve the KGB for 54 years. He passed over 900 British documents to the KGB. He served as a double agent. 
Of the five spies in the Cambridge Five spy ring, Philby was believed to have been the most successful at passing information to the Soviet Union.
In 1961, defector Anatoliy Golitsyn gave information which pointed to Philby. An MI6 officer, Nicholas Elliott, was sent in 1963 to interview him in Beirut. He reported that Philby seemed to know he was coming (this suggested the presence of yet another mole). Nonetheless, Philby allegedly confessed to Elliott.
Shortly afterwards, apparently fearing he might be abducted in Lebanon, Philby defected to the Soviet Union. He fled, under cover of night, to a Soviet freighter.