The Cambridge Five was a ring of British spies who passed information to the Soviet Union during World War II. The five consisted of Kim Philby, Donald Maclean, Guy Burgess, Anthony Blunt, and John Cairncross.

Cairncross's role in the spy ring was a mystery for decades. During this time, there was a great deal of speculation that Victor Rothschild was the fifth man. Indeed, speculation continues that Cairncross was a sixth man.