A cross-channel investigation has been launched after two submarines carrying nuclear weapons – from the Royal and French navies – collided in the Atlantic.

HMS Vanguard and Le Triomphant crashed on February 3 or 4, despite both vessels being equipped with sonar.

The British submarine, which has a crew of 135, weighs nearly 16,000 tonnes and is 150m long, is one of Britain's four V-class subs under the Trident programme.

Armed with 16 ballistic missiles, it is now believed to have been towed last night to Faslane, a nuclear submarine base in Scotland, with dents and scrapes to its hull.

Its French counterpart was reportedly carrying a similar number of warheads, and also had around 130 crew on board.

A senior Navy source was quoted in The Sun as saying that the potential consequences of such a collision would be "unthinkable".

"It’s very unlikely there would have been a nuclear explosion," the source told The Sun.

"But a radioactive leak was a possibility. Worse, we could have lost the crew and warheads. That would have been a national disaster."

The MoD refused to immediately confirm the incident had taken place, saying it was against its policy to comment on submarine operations.

However, a spokesman insisted nuclear security had not been breached. "We can confirm that the UK’s deterrent capability has remained unaffected at all times and there has been no compromise to nuclear safety," he said.

The French navy refused to confirm the accident, but a week ago it reported that Triomphant had returned to port after a collision at sea, "apparently with a container".

The 14,335-tonne vessel was ending a 70-day tour of duty when the incident took place at about the beginning of the week of February 1, the navy at Brest, its Breton home port, announced on February 6.

The collision took place while the vessel was submerged. Its fibreglass sonar dome, which protrudes from the bow of the vessel, was damaged and would take three or four months in dry dock to repair, the French navy said. It gave no other details except that there had been no injuries and no risk of accident at any time, spokesmen said on February 6.

Today, the Ministry of Defence has so far declined to say anything further about the Triomphant, which is one of four nuclear armed and powered submarines that patrol around the globe ensuring France's independent ability to mount a nuclear strike. All the nuclear submarines activities are covered by state secrecy.

In service since 1997, the Triomphant measures 455ft long. It is crewed by 112 men and armed with 16 M-45 missiles and four torpedo tubes. It operates at maximum speeds of 12 knots on the surface and 25 knots submerged.

France's Atlantic coast is known as a submarine graveyard because of the number of underwater craft sunk there during the Second World War – mainly German U-boats.

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