The Ministry of Defence has disclosed that Royal Navy divers undertook an unauthorised espionage operation against a Soviet warship docked in Portsmouth in 1955.

The “successful” covert mission to examine the sonar equipment fitted to the Soviet cruiser Sverdlov and other warships was six months before the notorious “Commander Crabb affair” in which a navy frogman vanished after being commissioned by MI6 to check the cruiser that brought Nikita Khrushchev on an official visit to Britain. A headless body, presumed to be that of Commander Lionel “Buster” Crabb, was washed ashore in Chichester 14 months later. It was one of the biggest spy dramas of the Cold War.

The disclosure of an earlier secret diving mission, in October 1955, has been made in a series of classified memorandums and letters released by the MoD under a freedom of information request. They relate to intensive efforts in Whitehall in the early 1970s and again in the 1980s to prevent a television producer from broadcasting a programme about the earlier diving mission, which he had uncovered while researching the Crabb story.

The Admiralty, backed by the Cabinet Office and Foreign and Commonwealth Office, told David Darlow, of Granada, that the programme would breach the national security guidelines set out by the D-notice system. The producer had approached the MoD for help in 1972. Rebuffed, he tried again for a programme to mark the 30th anniversary of Commander Crabb’s death. Despite the passage of time and the known facts about the mission to examine the underside of the Ordzhonikidze, which brought Khrushchev and Nikolai Bulganin, the Soviet Prime Minister, to Portsmouth in April 1956, it was decided that to assist Mr Darlow “would constitute a major breach of our longstanding policy of never discussing intelligence or security matters”.

A secret Whitehall memo added: “In this case, we would run the risk of confirming to the Soviet Union an event which they may still only suspect, and of thereby adding a further irritant to Anglo-Soviet relations.”

Confirmation of the previously unknown diving operation came in an Admiralty “background note” stamped “secret” and dated September 12, 1985, in which an official wrote: “In addition to the 1956 operation carried out against the Soviet ships by Commander Lionel Crabb, a separate diving operation was planned by the Royal Navy.

This second operation was officially cancelled before it took place, but there seems little doubt that some RN personnel from HMS Vernon [a shore-based establishment] nevertheless undertook a (successful) operation as a wholly unofficial enterprise.”

The papers on the MoD’s website include a letter sent to No 10 in 1981 by the naval officer who commanded the secret mission. His name is redacted but he wrote: “I was in charge of the naval operational team who successfully surveyed the undersides of the Russian ships at the time to ensure that all was either ‘safe’ or ‘unsafe’.” He added: “The Security Services, apart from alerting us to the need for this underwater survey operation, engaged Crabb on a separate mission which failed disastrously. This caused political chaos from the Prime Minister downwards.

“Rest assured that I have adhered to the confidentiality of this incident. There were others apart from myself who did the underwater work – I’m sure they too are loyal.”

In separate memos relating to the Crabb affair, questions and answers prepared by the Admiralty in 1956 reveal that the commander was not alone in his mission, and that it took place “without specific ministerial knowledge”.

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