Drawing on newly declassified intelligence documents and dozens of interviews with spymasters, The Defector tells a startling story of a Soviet mission to plant fake Kremlin agents within British and American intelligence services, the paranoia that ensued, and how the actions of a genuine turncoat, the former KGB officer Oleg Lyalin, and the secrets he revealed resulted to one of the most dramatic and pivotal moments in the Cold War.

Lyalin led MI5 to rethink its relationship with the CIA. And his defection discredited a previous KGB defector, Anatoly Golitsyn, the darling of the CIA, and ultimately destroyed the reputation of the US agency's head of counterintelligence, James Jesus Angleton.

As Richard Kerbaj writes: 'There was a poetic irony in Golitsyn's loss of credibility. It came, as he had previously feared, at the hands of a KGB defector. Except Oleg Lyalin had not been sent by the KGB - he was running away from it.'

At the heart of Lyalin's story is a narrative entwined with lies, disinformation, Kremlin deception campaigns, intelligence failures by the CIA and MI5, and a tangled love life. Told in full here, for the first time, by one of this country's leading commentators on national security, it reveals how during the darkest moments of the Cold War one of the West's greatest achievements transpired as a result of MI5's break with the CIA.

The disclosure of the inside story of this historic event also comes at a time when there is a renewed interest in the relationship between transatlantic spy services - from the intelligence they share or hold back, to the way they respond to their political masters and stand up to threats from Russia.