A Royal Navy submarine was sent on a Cold War mission to spy on its own side to prove that crews could safely carry out surveillance of the Russian fleet, a new book discloses.
The mission, known only to a handful of admirals, was designed to reassure Sir Winston Churchill that submarines could spy on Soviet vessels without being detected.
HMS Totem was able to spend 10 days creeping around supposedly alert British warships on an exercise off Gibraltar eavesdropping and taking photographs of their communications equipment without being detected.
The mission was so successful that the Prime Minister reversed his opposition to similar operations against Russian vessels and sanctioned spy missions off northern Russia.
The 1954 operation is disclosed in a new history of the Submarine Service which the authors say is written with unprecedented co-operation from one of the Armed Forces' most secretive branches.
The Silent Deep, by Peter Hennessy and James Jinks, details decades of deep-sea cat-and-mouse games waged between Royal Navy and Russian submarines at the height of the Cold War.
It also reveals that a Royal Navy submarine was able to track a Russian vessel armed with nuclear missiles for 49 days, setting what is believed to be a record for the longest trail of a Soviet submarine.
HMS Sovereign was able silently to follow the submarine in 1978 as it zig-zagged across the Atlantic, gathering vast quantities of intelligence about how the Soviet boat operated.
The book also contains new details of a 1968 collision between HMS Warspite and a Soviet Echo II cruise missile-carrying submarine which was hushed up for years.
The Royal Navy ship slammed into the back of the Russian submarine while it was tracking it and badly damaged its conning tower.
Hennessy, who went on several submarine voyages while writing the book, said he had wanted to tell the stories of people who had been unable to reveal their secret exploits at the time. "They are not called the silent service for nothing."