Fresh light could be shed on the mystery disappearance of a diver ordered to spy on a Russian cruiser.
Crabb was not the only diver to spy on the Ordzhonikdze
Secret files are released on Tuesday containing details of Commander Lionel "Buster" Crabb's fatal 1956 mission.
Crabb had been ordered to spy on the Ordzhonikdze in Portsmouth harbour after she brought Soviet Premier, Nikita Khruschev, to Britain for talks.
It is thought the papers contain confirmation that a second team was also sent to spy on the cruiser.
Although the mission was supposed to have been called off, it still went ahead as an "unofficial enterprise".
It is not known exactly what happened to Crabb, whose wartime exploits in the Mediterranean featured in the 1958 film, The Silent Enemy.
But a year after his disappearance, a body, thought to be Crabb's, was found in Chichester Creek with its head and hands cut off.
The papers released to the National Archives relate to Government attempts in the 1970s to dissuade the BBC from running a documentary on the affair.
In a note to Prime Minister Edward Heath, Cabinet Secretary Sir Burke Trend, wrote: "The BBC now know that, in addition to the operation by Crabb, a separate diving operation was planned by the Royal Navy against the Russian cruiser.
"They have also got wind of the fact this second operation, although officially called off, nevertheless took place as an unofficial enterprise."
The source for the story was said to be one of the divers involved.