The British government official spoke on condition of anonymity because of the sensitivity of the issue. The incident was first reported by a British newspaper, the Sun.
The HMS Vanguard, Britain's first Trident class nuclear-armed submarine, and the French Le Triomphant submarine, which was also carrying nuclear missiles, both suffered minor damage.
The British official said the Vanguard's "deterrent capability remained unaffected and there was no compromise to nuclear safety." No members of the crew were injured.
The HMS Vanguard, which went into operation in 1994, is one of Britain's four nuclear-powered submarines. Each is capable of carrying up to 16 nuclear-armed Trident missiles.
France's Defense Ministry said in a brief statement Feb. 6 that the Le Triomphant had struck "a submerged object (probably a container)" during a return from a patrol, damaging the sonar dome on the front of the submarine.
The French ministry did not confirm the date of the collision, nor did it make any mention of a British submarine.
It said in the statement that no crew members were injured and confirmed that the nuclear security of the submarine was not compromised.
After the accident, the French submarine returned to its base on L'Ile Longue on France's western tip under its own power, escorted as usual by a frigate, the ministry said.
The incident sparked concern among nuclear activists, who have long warned that nuclear submarines pose risks of radioactive leaks into the world's waters.
"This is a nuclear nightmare of the highest order," said Kate Hudson, chair of Britain's Campaign for Nuclear Disarmament. "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."
She called on British Prime Minister Gordon Brown to end patrols, which she said have at least one British nuclear sub in the Atlantic at all times.
Stephane Lhomme, a spokesman for the French anti-nuclear group Sortir du Nucleaire, said its network of activists is on alert for any signs of radioactive leaks near French shores.
"This reminds us that we could have a new catastrophe with a nuclear submarine at any moment. It is a risk that exists during missions but also in port," he said. "These are mobile nuclear reactors."