London ? Nuclear submarines from Britain and France collided deep in the Atlantic Ocean this month, authorities said Monday in the first acknowledgment of a highly unusual accident that one expert called the gravest in nearly a decade.

Officials said 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.

France’s defense ministry said Monday that the sub Le Triomphant and the HMS Vanguard, the oldest vessel in Britain’s nuclear-armed submarine fleet, were on routine patrol when they collided in the Atlantic this month. It did not say exactly when, where or how the accident occurred.

France said that Le Triomphant suffered damage to a sonar dome — where navigation and detection equipment is stored — and limped home to its base on L’Ile Longue on France’s western tip. HMS Vanguard returned to a base in Scotland with visible dents and scrapes, the BBC reported.

“The two submarines came into contact at very low speed,” Britain’s First Sea Lord, Admiral Jonathon Band, said. Band, Britain’s most senior naval officer, offered no further explanation.