A Royal Navy submarine packed with nuclear ballistic missiles crashed into a French vessel in the middle of the Atlantic, it emerged today.

The collision between HMS Vanguard and French submarine Le Triomphant, which was also carrying nuclear weapons, could have released deadly radioactive material into the ocean.

Defence sources said the vessels, which between them had 250 sailors on board, could have sunk with a devastating loss of life.

The submarines are both equipped with stealth anti-sonar technology, which was apparently so effective that neither submarine realised the other was in the same part of the sea.

A senior Navy source told The Sun: "The potential consequences are unthinkable. It's very unlikely there would have been a nuclear explosion.

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

The crash, which was believed to have taken place on 3 or 4 February, occurred when both submarines were submerged and on separate missions.

HMS Vanguard was towed into the nuclear submarine base Faslane in the west coast of Scotland last night, reportedly with visible dents and scrapes on its hull.

Le Triomphant was taken to Brest in north-western France with extensive damage to its sonar dome.

The Ministry of Defence refused to confirm that the collision had taken place, saying it was against its policy to comment on submarine operations.

However, a spokesman insisted that nuclear security had not been breached.

He said: "We can confirm that the UK's deterrent capability has remained unaffected at all times and there has been no compromise to nuclear safety."

HMS Vanguard, which weighs nearly 16,000 tonnes, is 150 metres long and has a crew of 135. It is one of Britain's four V-class submarines forming the Trident nuclear deterrent.

Each is armed with 16 ballistic missiles carrying up to eight warheads .

French Navy sources confirmed that Le Triomphant, one of four strategic nuclear submarines of the so-called "Force de Frappe", was returning from a 70-day tour of duty when the collision occurred.

The French were trying to play down the crash. A French Navy spokesman said: "The collision did not result in injuries among the crew and did not jeopardise nuclear security at any moment."

Inquiries are underway on both sides of the Channel. One military source said: "The lines between London and Paris have been hot."

France's Atlantic coast is notorious for being a submarine graveyard because of the number of underwater craft, mainly German U-boats, sunk in the area during the Second World War.

The mid-Atlantic collision is the biggest embarrassment for the Royal Navy since Iran captured 15 sailors in 2007.

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