"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."