LONDON (Reuters) - 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 on Monday.
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 the 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, newspapers reported. 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 to 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 (66lb to 110lb) 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. "Therefore it's not surprising that they couldn't hear each other.
"In the history of UK nuclear powered submarine operations, this is the first time something like this has ever happened."
After the accident in the mid-Atlantic, the Royal Navy's HMS Vanguard returned to base in Faslane, western Scotland, with dents and scrapes visible on its hull, the Sun reported.
The Scottish National Party (SNP) lawmaker Angus Robertson said Britain's defence ministry "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."
The Campaign for Nuclear Disarmament said it was the most severe incident involving a nuclear submarine since the sinking of the Russian Kursk submarine in 2000.
"This is a nuclear nightmare of the highest order," said CND Chair Kate Hudson. "The collision of two submarines, both with nuclear reactors and nuclear weapons onboard, could have released vast amounts of radiation and scattered scores of nuclear warheads across the seabed."
Launched in 1992, Vanguard is one of four British submarines that carries the Trident nuclear missile, the country's nuclear defence system. At least one is on patrol at all times.
Le Triomphant, which entered service in 1997, carries 16 nuclear missiles and is one of four nuclear-armed submarines in the French fleet.