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.

The first public indication of a mishap came when France reported in a little-noticed Feb. 6 statement that one of its submarine 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 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 submarine base in Scotland with visible dents and scrapes, the BBC reported.

HMS Vanguard came into service in 1993, has a crew of around 140 and typically carries 16 Lockheed Trident D5 missiles. Under government policy, British nuclear submarines carry a maximum of 48 warheads. At least one of Britain’s four submarines is on patrol and ready to fire at any given time.

France’s Le Triomphant carries 111 crew and 15 nuclear missiles, according to defense analysis group Jane’s.

“It’s an absolute one in a million chance that the two submarines were in the same place at the same time,” said Lee Willett, head of the maritime studies program at the Royal United Services Institute, a London-based military think tank. “There is no precedent of an incident like this — it’s a freak accident,” he said.

Stephen Saunders, a retired British Royal Navy commodore and the editor of Jane’s Fighting Ships, said that while NATO countries let each other know what general area of the Atlantic they are operating in, neither submarine would have had a precise position for the other.

“This really shouldn’t have happened at all,” Saunders said. “It’s a very serious incident, and I find it quite extraordinary.”

Both Saunders and Willett said submarines don’t always turn on their sonar systems, or make their presence obvious.

“The whole point is to go and hide in a big chunk of ocean and not be found. They tend to go around very slowly and not make much noise,” Saunders said.