A Royal Navy nuclear submarine was involved in a collision with a French nuclear sub in the middle of the Atlantic, the MoD has confirmed.

HMS Vanguard and Le Triomphant were badly damaged in the crash in heavy seas earlier this month.

First Sea Lord Admiral Sir Jonathon Band said the submarines came into contact at low speed and no injuries were reported.

Both the UK and France insisted nuclear security had not been compromised.

BBC defence correspondent Caroline Wyatt said the incident was "incredibly embarrassing" for the Ministry of Defence (MoD).

She said HMS Vanguard, with "very visible dents and scrapes", had to be towed back into its home base at Faslane on the Firth of Clyde.

The submarines are equipped with sonar to detect other vessels nearby but our correspondent said it might the case that the anti-sonar devices, meant to hide the submarines from enemies, were "too effective".

"This is clearly a one-in-a-million chance when you think about how big the Atlantic is," she said.

Sonar dome

The two submarines are key parts of each nation's nuclear deterrent, and would have been carrying missiles, though both the UK and France have insisted there was no danger of a nuclear incident.

The two submarines were carrying around 240 sailors between them. A French naval spokesman said the collision did not result in any injuries to the crew.

Le Triomphant is based at L'Ile Longue near Brest, north-west France. HMS Vanguard arrived back in Faslane on Saturday.

France's defence ministry said in a statement, dated 6 February, that Le Triomphant "collided with an immersed object (probably a container)" when coming back from patrolling.

"The sonar dome, at the front, was damaged. This incident did not cause any injuries among the crew and did not threaten the nuclear security at any time," it said.

"The submarine came back by its own means to L'Ile Longue, escorted by a frigate, as it is the usual practice when leaving or coming back."

'Large ocean'

Nuclear engineer John Large told the BBC that navies often used the same "nesting grounds".

"Both navies want quiet areas, deep areas, roughly the same distance from their home ports. So you find these station grounds have got quite a few submarines, not only French and Royal Navy but also from Russia and the United States."

Politicians have demanded more information on the incident.

Lib Dem defence spokesman Nick Harvey has called for an immediate internal inquiry with some of the conclusions made public.

"While the British nuclear fleet has a good safety record, if there were ever to be a bang it would be a mighty big one," he said.

"Now that this incident is public knowledge, the people of Britain, France and the rest of the world need to be reassured this can never happen again and that lessons are being learned."

Meanwhile, SNP Westminster leader Angus Robertson has called for a government statement.

"The Ministry of Defence needs to explain how it is possible for a submarine carrying weapons of mass destruction to collide with another submarine carrying weapons of mass destruction in the middle of the world's second-largest ocean," he said.

The Campaign for Nuclear Disarmament described the collision as "a nuclear nightmare of the highest order".

CND chair Kate Hudson said: "The collision of two submarines, both with nuclear reactors and nuclear weapons onboard, could have released vast amounts of radiation and scattered scores of nuclear warheads across the seabed."