Kim Philby, the so-called Third Man in the Cambridge spy ring, was the Cold War's most infamous traitor, a Soviet spy at the heart of British intelligence. Philby joined MI6 during the war. He betrayed hundreds of British and US agents to the Russians and compromised numerous operations. Tim Milne was Philby's closest friend and his treachery was a huge blow. His revealing description of Philby's time in MI6 was banned, but his family were determined it should be published. The final word on the notorious spy by the MI6 colleague who knew him best