Nuclear submarines from Britain and France collided deep in the Atlantic Ocean earlier in February, authorities said Feb. 16, in the first acknowledgment of a highly unusual accident that one expert called the gravest in nearly a decade.

HMS Vanguard sits in Holy Loch, Scotland, in 1992. AP PHOTO

Officials said that the low-speed crash did not damage the vessels' nuclear reactors or missiles, or cause radiation to leak. But anti-nuclear groups said it was still a frightening reminder of the risks posed by submarines prowling the oceans powered by radioactive material and bristling with nuclear weapons.

The first public indication of a mishap came when France reported in a little-noticed Feb. 6 statement that one of its submarines had struck a submerged object — perhaps a shipping container. But confirmation of the accident only came after British media reported it.

France's Defense Ministry said Feb. 16 that the submarine Le Triomphant and HMS Vanguard, the oldest vessel in Britain's nuclear-armed submarine fleet, were on routine patrol when they collided in the Atlantic. It did not say exactly when, where or how the accident occurred.

France said that Le Triomphant suffered damage to a sonar dome and limped home to its base on L'ile Longue on France's western tip. HMS Vanguard returned to a submarine base in Scotland with visible dents and scrapes, the BBC reported.

"This is the most severe incident involving a nuclear submarine since the sinking of the Kursk (a Russian vessel) in 2000 and the first time since the Cold War that two nuclear-armed subs are known to have collided," said Kate Hudson, head of Britain's Campaign for Nuclear Disarmament.