In a first-ever such accident, a British and a French nuclear submarine, both carrying nuke warheads, collided in the Atlantic Ocean earlier this month, but there was no loss of life.
HMS Vanguard, a British 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 damage to the nuclear reactor or weapons, the Sun newspaper reported on Monday.
The British sub has now been towed to Faslane in Scotland for repairs. Though there was no reports of the French vessel, 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 refused to comment on the 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 the four nuclear submarines operated by the British military as part of its Trident system, and one is always on a 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 the warheads. That would have been a national disaster," the source added.
As inquiries began, naval sources said the collision was a millions-to-one unlucky chance as both the 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 that neither of the submarines "saw" the other.
The British defence ministry insisted on Sunday night that there had been no nuclear security breach. But definitely this is the biggest embarrassment to the British Navy since Iran captured 15 sailors in 2007.