Politicians and campaigners yesterday demanded an inquiry into the failings that allowed two nuclear submarines to collide in the middle of the Atlantic Ocean.

HMS Vanguard crashed into a French counterpart earlier this month, the Ministry of Defence (MoD) was forced to admit, calling into question the safety and management of Britain's nuclear arsenal.

The Navy is now under pressure to explain how the incident - branded "embarrassing" by a leading military analyst - was ever allowed to happen. Unofficial reports said both HMS Vanguard and Le Triomphant were carrying nuclear warheads and the British vessel was understood to have visible damage when it returned to its base at Faslane on the Clyde.

The First Sea Lord, Admiral Sir Jonathon Band, worked to quash public fears over the incident, stating that no-one was injured and the submarine had been able to return to base at Faslane unescorted and with no serious problems.

A spokesman for the Prime Minister added: "It is not our policy to comment on submarine operational matters but we can confirm that the UK's deterrent capability remains unaffected and there has been no compromise to nuclear safety."

However, opposition spokesmen said the crash exposed serious failings in the management and policy governing Britain's nuclear fleet.

Angus Robertson, the SNP's Westminster leader and defence spokesman, 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.

Shadow defence secretary Dr Liam Fox called the incident "extremely worrying", and Liberal Democrat defence spokesman Nick Harvey called for an urgent internal inquiry with public conclusions.

He 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."

The MoD indicated that some sort of internal investigation would now be launched.

Leaders of the Campaign for Nuclear Disarmament described the crash as "a nuclear nightmare of the highest order", adding that it was "the most severe incident involving a nuclear submarine since the sinking of the Kursk in 2000".

John Ainslie, co-ordinator of Scottish CND, said: "This highlights the danger of keeping nuclear submarines at sea when there is no threat to Britain. Trident is a danger to us all. At the very least these vessels should immediately be confined to base. The government's plan to spend billions on new Trident submarines should be scrapped."

Commodore Stephen Saunders (retired), editor of military magazine Jane's Fighting Ships, said the crash was most likely caused by a combination of procedural failings and simple bad luck.

"The reported collision between HMS Vanguard and Le Triomphant is a very serious incident," he said.

"As far as I am aware, it is the first time that the submarines of two friendly nations have been involved in such an accident." He added: "It is worth noting that submarines are robustly built. USS San Francisco ran into an underwater seamount at high speed in 2005 but survived. In today's case, two large submarines hit each other, probably at low speed, and the damage, while embarrassing, can be repaired."