British Trident submarine is returning to its home port after a mechanical failure led to loss of power while on a training exercise, the Ministry of Defence said today

HMS Vengeance, one of the Royal Navy's four nuclear-powered Vanguard-class submarines which comprise Britain's nuclear "deterrent," is returning on the sea's surface to Faslane naval base, Scotland.

An MoD spokesman said: "Vengeance has suffered a mechanical defect resulting in a reduction in propulsion. She is returning to Faslane under her own power. She is still at sea."

The spokesman was unable to confirm reports that the submarine's propulsion unit had become fouled by sea debris.

The incident happened on Thursday night while the vessel was on a training exercise in the north Atlantic, he said.

The boat carries up to 48 nuclear warheads on up to 16 Trident missiles, which weigh 60 tonnes each and have a range of 4,000 nautical miles.