Two nuclear-armed submarines from Britain and France collided while on separate patrols in the Atlantic Ocean but there were no injuries or radioactive leaks, naval officials said yesterday.

Analysts said a major disaster could have resulted had the underwater collision ruptured the hulls, set off conventional ammunition or started a fire, although the chances of a full nuclear explosion were virtually nil.

The nuclear-powered submarines collided earlier this month but there was no damage to the vessels' weapons, said First Sea Lord Admiral Jonathon Band, head of Britain's Royal Navy.

British and French officials have so far failed to explain how two sophisticated vessels from allied nations could collide in open water, a highly unusual event that is deeply embarrassing for both navies.

"The submarines came into contact at very low speeds, both submarines remain safe and no injuries occurred," he told a news conference in London.

"There was no compromise to nuclear safety."

Both submarines were badly damaged and had to return to port, British newspapers reported. Lord Admiral Band and the British and French defence ministries would not comment on those reports.

The French navy issued a statement earlier this month saying its submarine, Le Triomphant, had hit a "submerged object, probably a container". The impact damaged its sonar dome and it had to return to France for repairs.

It later became clear it had actually collided with the British submarine.

"This incident caused no crew injuries and at no time threatened nuclear security. There has been no interruption to the nuclear deterrent capability," the French defence ministry said in a statement.

Independent nuclear analyst John Large, who advised the Russian government after its Kursk submarine sank in 2000, said the incident could have been far worse.

"The real risk is if you have a fire on board caused by the impact," he said. "Each warhead has about 30-50 kg of high explosive around it. That would burn and your plutonium core would burn as well. That would disperse into the atmosphere and be a major problem."

They were in the same part of the ocean because both submarines would have had similar targets and needed to be within range of their bases in western Europe, he added.

Lee Willett, of the Royal United Services Institute for Defence and Security Studies, said the "freak occurrence" was in part due to the submarines' stealthy design.

"The whole point of a deterrent submarine is that it is as quiet as possible so you can't find it. They are meant to be like holes in the water," he said.