A Royal Navy nuclear submarine and a French vessel have been damaged in a collision deep below the surface of the Atlantic Ocean.

HMS Vanguard and Le Triomphant, which were carrying nuclear missiles on routine patrols, are reported to have collided while submerged on 3 or 4 February. Between them they had about 250 sailors on board.

The Ministry of Defence initially refused to confirm the incident, saying it was not policy to comment on submarine operations. This afternoon the First Sea Lord, Admiral Sir Jonathon Band, issued a statement saying the two vessels hit each other while travelling at very low speeds and no one was injured.

"We can confirm that the capability remained unaffected and there has been no compromise to nuclear safety," he said. The MoD said the Vanguard returned to its base in Faslane, Scotland, with only "scrapes".

Defence officials told theguardian.com the two submarines collided in what they said was an extraordinary accident. "They can't see each other in the water," one official said, raising questions about the submarines' sonar and why they did not detect one another.

Opposition parties asked how the accident was possible. The SNP's Westminster leader, Angus Robertson, said: "The UK Ministry of Defence needs to explain how it is possible for a submarine carrying weapons of mass destruction to collide with another submarine carrying weapons of mass destruction in the middle of the world's second-largest ocean.

"In contrast to MoD secrecy, the French military authorities publicised details of the incident on a website. The MoD cannot hide behind operational secrecy and must make a statement on this as a priority."

The shadow defence secretary, Liam Fox, called the incident "extremely worrying".

The Liberal Democrat defence spokesman, Nick Harvey, said: "While the British nuclear fleet has a good safety record, if there were ever to be a bang it would be a mighty big one. The public entrust this equipment to the government confident that all possible precautions are being taken. Now that this incident is public knowledge, the people of Britain, France and the rest of the world need to be reassured this can never happen again and that lessons are being learned."

France's defence ministry said in a brief statement on 6 February that the Triomphant had struck "a submerged object (probably a container)" during a return journey from a patrol, damaging the sonar dome on the front of the submarine.

It said no crew members were injured and the nuclear security of the submarine had not been compromised.

Today the ministry confirmed that another sub was involved, saying: "They briefly came into contact at a very low speed while submerged."

After the accident, the French submarine returned to its base on L'Ile Longue, near Brest, under its own power and escorted by a frigate.

Vanguard, one of Britain's four V-class submarines that make up the Trident nuclear deterrent, each of which is capable of carrying up to 16 missiles, was said to have visible dents on its hull as it was towed home at the weekend. Inquiries are under way on both sides of the Channel.

Kate Hudson, the Campaign for Nuclear Disarmament chairwoman, described the incident as "a nuclear ­nightmare of the highest order".

"The collision of two submarines, both with nuclear reactors and nuclear weapons on board, could have released vast amounts of radiation and scattered scores of nuclear warheads across the seabed," she said. "The dents reportedly visible on the British sub show the boats were no more than a couple of seconds away from total catastrophe."

Hudson said it was the first time since the cold war that two nuclear-armed submarines were known to have collided.

"These dangers are inherent whilst the British government maintains its 1960s policy of having at least one nuclear ­weapons submarine sailing round the Atlantic 24 hours a day, 365 days a year," she said. "HMS Vanguard is likely to be confined to port for months with a multimillion-pound repair bill. Gordon Brown should seize this opportunity to end continuous patrols."

Le Triomphant, which entered service in 1997, carries 16 nuclear missiles and is one of four nuclear-armed submarines in the French fleet.

Stephane Lhomme, a spokesman for the French anti-nuclear group Sortir du Nucleaire, said its network of activists was on alert for any signs of radioactive leaks near French shores.

"This reminds us that we could have a new catastrophe with a nuclear submarine at any moment. It is a risk that exists during missions but also in port," he said. "These are mobile nuclear reactors."