Guy Liddell was the Director of MI5’s counter-espionage B Division throughout the Second World War, during which he wrote a confidential personal diary. Within its pages details of virtually every important event that had any intelligence significance during the conflict were recorded.

Those recently declassified diaries, which were edited by Nigel West, have been followed by a postwar series which cover the period from the German surrender until Liddell’s sudden resignation in May 1953. These eight years of the early Cold War contain many disturbing secrets, such as the cache of incriminating Nazi documents which was supposed to be destroyed by the SS. When these were recovered intact the British government went to considerable lengths to keep their contents from being disclosed, for they provided proof of the Duke of Windsor’s contact, through a Portuguese intermediary, with the enemy during the crucial period in 1940 when the ex-king declared himself ready to fly back from the Bahamas and be restored to the throne. One of Liddell’s first tasks, at the request of Buckingham Palace, was to retrieve and suppress the damaging material.

Liddell’s diaries were never intended for publication and are therefore filled with indiscretions that shed new light on MI5 investigations that he supervised after his promotion to Deputy Director-General.

Many in Whitehall anticipated that Liddell would become Director-General but, as these pages reveal, he had employed Anthony Blunt as his trusted personal assistant, had found it hard to accept the evidence of Kim Philby’s treachery, and had maintained an unwise friendship with Guy Burgess. Nevertheless, despite Liddell’s manifest failings, and his reluctance to believe in the disloyalty of men he regarded as friends, he was probably the single most influential British intelligence officer of his era.