Maclean, Burgess, Philby and Blunt were British members of a KGB spy ring that penetrated the intelligence system of the UK and passed vital information to the Soviets during World War Two and the early stages of the Cold War.

The members of the ring were Donald Maclean (1913 - 1983), Guy Burgess (1911 - 1963), Harold 'Kim' Philby (1912 - 1988) and Anthony Blunt (1907 - 1983). Several other people have been suggested as belonging to the ring, including John Cairncross. Blunt became a communist in the early 1930s and was recruited by the NKVD (later KGB), the Soviet security agency. While teaching at Cambridge University, Blunt was influential in recruiting the other three, who were all students there.

Burgess became a journalist after he left university, but on the outbreak of war joined MI6. Maclean was in the Foreign Office during the same period. In 1951, tipped off by Philby that they were under suspicion, Burgess and Maclean defected to the Soviet Union, where they spent the rest of their lives.

Philby was also a journalist but joined SIS (also known as MI6) in 1940. Just before the war ended, he was appointed head of SIS's anti-Soviet section. Thus the man charged with running operations against the Soviets was a KGB agent. He later became chief British intelligence officer in the United States. After Burgess and Maclean fled to the Soviet Union, Philby came under suspicion and was forced to resign. In 1963, he defected to the Soviet Union, and died there. Blunt worked for MI5 during the war. After the war he had a distinguished career as an art historian. He was director of the Courtauld Institute and Surveyor of the Queen's Pictures. He was knighted in 1956. In 1963, the British government discovered he was a spy but offered him immunity in return for information. In 1979, the story got out and Blunt was stripped of his knighthood.