HMS Vanguard, a Royal Navy's Trident class, and French Navy's new Le Triomphant class submarines were both damaged in the incident, but there were no reports of casualties or damages to the nuclear reactor or weapons, the Sun newspaper reported on Monday.

British sub has now been towed to Faslane in Scotland for repair. Though there was no reports of the French vessel, while, Triomphant limped to Brest with extensive damage to her sonar dome, the paper said.

Both vessels – between them carrying about 250 sailors – were reportedly submerged and on separate missions when they crashed on February 3 or 4.

The British Ministry of Defence refuses to comment on submarine operations but a spokesman 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 is one of four nuclear submarines operated by the British military as part of its Trident system, and one is always on deterrent patrol.

Each vessel is 150 metres long and 13 metres in diameter, and can carry up to 48 nuclear warheads including 16 missiles.

"The potential consequences are unthinkable. It's very unlikely there would have been a nuclear explosion," a senior Royal Navy source said.

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

As inquiries began, naval sources said it was a millions-to-one unlucky chance both subs were in the same patch of sea. Warships have sonar gear which locates submarines by sound waves.

But modern anti-sonar technology is so good it is possible neither boat ‘saw’ the other.

"The lines between London and Paris have been hot," a senior military source said.