A former Russian frogman has claimed that he killed British diver Cdr Lionel "Buster" Crabb, who disappeared while spying on a Soviet warship in 1956.
"Buster" Crabb had been awarded the George Cross
Cdr Crabb vanished after the vessel, which had brought Soviet leaders to Britain, docked in Portsmouth Harbour.
Now retired sailor Eduard Koltsov claims he cut the Englishman's throat as he caught him placing a mine.
Mr Koltsov tells a Russian documentary that he needed to tell the truth about the Cold War mystery before he died.
The Soviet ship Ordzhonikidze had brought Nikita Khrushchev and other leaders for meetings with the British prime minister Anthony Eden and his ministers.
Several months later a headless corpse, identified by a friend as Cdr Crabb, was found floating along the coast.
At the time the diver went missing, the Navy said he was feared drowned in Stokes Bay - some miles to the west of Portsmouth Harbour.
What happened to him had been a mystery ever since.
But now Mr Koltsov, who was 23 at the time of the incident, says that he had been ordered to investigate suspicious activity around the ship.
He says he then spotted Cdr Crabb fixing the mine on the ship's hull.
In the documentary, he shows what he says is the dagger he used and the Red Star medal he says he was later secretly awarded for his bravery.
"I saw a silhouette of a diver in a light frogman suit who was fiddling with something at the starboard, next to the ship's ammunition stores," he tells the film crew.
The Ordzhonikidze brought Nikita Khrushchev on a diplomatic visit
"I swam closer and saw that he was fixing a mine."
Cdr Crabb - who was 47 when he disappeared - had been well-known for his actions in World War II. He received the George Medal for removing Italian limpet mines from British warships at Gibraltar and an OBE for mine clearance at Livorno.
The incident wrecked attempts at a rapprochement between Britain and the post-Stalin government in Moscow.
The Russians protested they were being spied upon by their hosts and, in the Commons, the government was asked if the security services were out of control.