HMS Vengeance had temporary 'defect in propulsion', a mechanical failure possibly due to ingesting sea debris
A nuclear-powered submarine of the Royal Navy has returned to its home port after a mechanical failure led to a loss of propulsion while training in the Atlantic.
HMS Vengeance, one of the four Vanguard class submarines making up Britain's nuclear deterrent, was later able to sail back to Faslane naval base on Sunday morning.
A Ministry of Defence spokesman said the incident was "not nuclear related" but had been due to "a mechanical defect resulting in a reduction in propulsion".
He was unable to confirm reports that the propulsion unit had become fouled by sea debris. It happened on Thursday night while on an exercise in the north Atlantic.
The submarine carries a crew of 141 and up to 16 Trident missiles with up to 48 nuclear warheads, as well as conventional torpedoes.
The submarine is powered by an enclosed set of propulsion blades at the stern resembling a turbofan jet engine.
John Large, a consultant on nuclear safety, told Scotland's Sunday Herald newspaper that the incident would have been "a scary moment for the crew".
"There will be red faces in the navy about this," he added. "One of our top-secret stealth submarines was suddenly crippled at sea, making her a lame sitting duck on the surface."
He suggested that the propulsor could have been fouled by discarded fishing gear, or perhaps a steel hawser. "It would have to be something quite heavy, and it could cause serious damage."
The Royal Navy held an inquiry last year into why HMS Astute, the world's most advanced submarine, ran aground in off the Isle of Skye.
Another Trident submarine, HMS Vanguard, collided with a French nuclear missile submarine, Le Triomphant, under the western Atlantic; both were seriously damaged.
In 2007, two sailors were killed by an explosion in an emergency oxygen generator on board submarine HMS Tireless during an exercise under the Arctic.