A British official said nuclear-armed submarines from Britain and France collided deep under the Atlantic Ocean while on separate exercises earlier this month, but limited details are only now emerging.
Britain's First Sea Lord, Admiral Jonathon Band made the disclosure after the British sub, HMS Vanguard returned to its home port of Faslane, in western Scotland.
"A few days ago, the submarines came into contact at very low speed. Both submarines remained safe. No injuries occurred. We can confirm the capability remain unaffected and there was no compromise to nuclear safety," he said.
Britain's Sun newspaper reported the Vanguard returned with dents and scrapes visible on its hull.
The French defense ministry said its vessel, Le Triomphant, sustained damage to is sonar dome, but it was able to return safely, under it own power, to its base in Brittany and no one on board was injured.
While such collisions are uncommon, nuclear expert John Large said such boats often ply the same waters.
"Both navies want quiet areas, deep areas, roughly the same distance from their home ports. So, you find that these nesting grounds, these station grounds, are pretty, have got quite a few submarines. Not only the French and Royal Navy submarines, but also from Russia and from the United States," he said.
The subs can carry up to 16 nuclear missiles each and both countries have four-vessel nuclear-armed sub fleets.