Bill Haydon, with his easy charm and strong social connections, bears a close resemblance to real-life double agent Kim Philby whose defection to the USSR in 1963, and the consequent compromising of British agents, was a factor in the 1964 termination of Le Carré's own career in the SIS. Philby, along with Guy Burgess, Donald Maclean, Anthony Blunt, and John Cairncross, was part of a group of Soviet moles in Britain which later came to be known as the Cambridge Five. While in an earlier draft of the novel, there were reportedly greater similarities to Anthony Blunt, the final character is closely aligned with Philby.