The admission ends more than 30 years of silence over one of the most elaborate and expensive projects of the Cold War.

The CIA has always refused to confirm even the barest details of Project Azorian, a daring 1974 exercise that was backed by the industrialist Howard Hughes and estimated to have cost £1 billion in today's money.

However, following an application to declassify the information under the US Freedom of Information Act, the CIA has released an internal account of the mission, albeit with some of the biggest mysteries still unanswered.

In the 50-page article published in 1985 in the agency's in-house journal, the CIA details how President Nixon went against the advice of his senior military chiefs in the hope of gaining crucial intelligence from the nuclear missiles being carried by the sub.

The Soviet Golf-II sub, the K-129, sank in 1968 in the Pacific, northwest of Hawaii, in circumstances that have never been explained.

It was carrying three ballistic missiles armed with nuclear warheads. According to the newly-released papers, despite the difficulties of reaching the vessel some three miles down, Richard Nixon ordered the creation of a task force to bring it to the surface.

The project was nearly cancelled due to soaring costs and concern that it might damage improving US-Soviet relations.

However, a portion of the sub was eventually winched to the surface by the Hughes Glomar Explorer, a specially-designed salvage ship using a unique lifting cradle.

Mr Hughes lent his name to the project to give the ship cover as a deep-sea mining vessel but the CIA papers reveal that she was continually dogged by Soviet ships.

Fearing the Russians might even try to storm the ship, the Americans blocked up its helicopter landing pad with crates.

The Americans buried six lost Soviet mariners at sea, after retrieving their bodies in the wreckage.

Exactly what the operation managed to salvage remains unclear as portions of the CIA text have been redacted, but historians and journalists have concluded that the most sensitive Soviet equipment was never recovered The CIA article – obtained by the National Security Archive, an independent watchdog – mentions only "intangibly beneficial" results such as the morale boost it gave to US intelligence and advances in maritime heavy-lifting technology.