Beirut, 1963: Nicholas Elliott is sent to interrogate double agent Kim Philby. Public school, Cambridge and MI6, they have been friends for 30 years. But that friendship is over.

With access to newly released MI5 files and previously unseen family papers, and with the cooperation of former officers of MI6 and the CIA, author Ben Macintyre unlocks the last great secret of the Cold War.

Kim Philby was the most notorious British defector and Soviet mole in history. Agent, double agent, traitor and enigma, he betrayed every secret of Allied operations to the Russians in the early years of the Cold War. Philby's two closest friends in the intelligence world, Nicholas Elliott of MI6 and James

Jesus Angleton the CIA intelligence chief, thought they knew Philby better than anyone - only to discover they had not known him at all.

This is a story of intimate duplicity; of loyalty, trust and treachery, class and conscience; of an ideological battle waged by men with cut-glass accents and well-made suits in the comfortable clubs and restaurants of London and Washington; of male friendships forged, and then systematically betrayed.

The story begins in Beirut 1963, a bugged room. Two men who've known each other for thirty years face one another. Both are spies, but one is a traitor. The first episode of A Spy Among Friends starts with Kim Philby's confession to his best friend.

Read by: Simon Russell Beale

Producer: David Roper

A Heavy Entertainment production for BBC Radio 4.