The last great mystery of Britain's Cold War

The frogman never returned from an unauthorised spying mission on a Russian cruiser carrying Nikita Krushchev to Britain on an official visit 50 years ago. The British government was left gravely embarrassed when the mission's failure became public knowledge, after a bungled operation to cover it up by the intelligence service.

Now the full extent of Anthony Eden's fury is to be laid bare when a long-suppressed report into the affair which was sent to the then Prime Minister from his most senior civil servant is published this week under Freedom of Information legislation. But while Sir Edward Bridges confirms that Eden specifically refused permission for MI6 to carry out underwater surveillance, it fails to answer the key question of what happened to Crabb.

A former colleague has denied that a headless corpse found a year later was that of the frogman spy, a war hero reported to have fallen on hard times. It has been variously claimed that Crabb defected, was captured and interrogated or that a Russian sniper who spotted him inspecting the cruiser's propeller in Portsmouth harbour shot the diver.

Sir Edward's report - withheld until now, first on the grounds of national security, and then with the excuse that the papers might have been contaminated with asbestos - is still heavily censored. Marked "Top Secret", it details how Crabb was taken by a naval officer to Portsmouth and made a practice dive, in which he appeared "in good trim", but disappeared the next day on his mission.

The papers detail how MI6 desperately sought to keep the failed mission secret. As Crabb's employer began asking awkward questions about his whereabouts, his name was removed by local police from the register of the Portsmouth hotel in which he had been staying. When the story began to break about the famous frogman's disappearance, senior officials in the Admiralty, the Foreign Office and MI6 argued among themselves about who was responsible, agreeing only not to tell any ministers.

A key section of the report on why - in the Cabinet Secretary's words - "due precautions were not taken in carrying out this operation" has been blanked out. Historians may be consoled, however, by ample evidence of Eden's anger at the botched mission, littered in the margins of the report. Against one passage explaining how Whitehall mandarins agreed to keep the affair from their various ministers, Eden scribbled, "Ridiculous".

And against another, admitting that the Russian captain told his British opposite number just three days after the operation that a frogman had been spotted, the British leader wrote: "We were never told."

His fury is explained in a chronology of the affair included in the report. It shows that on 12 April 1956 - just one week before Crabb's disastrous dive - Eden explicitly rejected a request to mount an underwater surveillance operation. "I am sorry, but we cannot do anything of this kind on this occasion," he told the First Lord of the Admiralty.