Two British and French nuclear submarines collided in heavy seas in the Atlantic. By Aislinn Simpson Last Updated: 7:02AM GMT 16 Feb 2009 The Guardian HMS Vanguard and Le Triomphant are understood to have both been severely damaged in the underwater accident earlier this month. Both are fitted with state-of-the-art technology aimed at detecting other submarines, but it apparently failed completely. Although both France and Britain insist that security was not compromised during the collision and there was no danger of a nuclear incident, inquiries are now under way in both countries. Each boat is a key part of their respective county's nuclear deterrent, ready to unleash their destructive weapons at a moment's notice. French Navy sources confirm that Le Triomphant, one of four strategic nuclear submarines of the so-called "Force de Frappe", was returning from a 70 day tour of duty when the incident occurred. It happened in heavy seas, and in the middle of the night between February 3 and 4, and left Le Triomphant's sonar dome all but destroyed. The sonar dome should have detected the Vanguard but Le Triomphant's crew of 101 claimed to have "neither saw nor heard anything". The French tried to play down the collision, with a Navy spokesman saying: "The collision did not result in injuries among the crew and did not jeopardise nuclear security at any moment." The Ministry of Defence would not even confirm it had taken place. A spokesman said: "It is MoD policy not to comment on submarine operational matters, but we can confirm that the UK's deterrent capability has remained unaffected at all times and there has been no compromise to nuclear safety." Le Triomphant took at least three days to limp back to her home port, while HMS Vanguard returned to her home base in Faslane, in Scotland. With a complement of 135 crew, she is the lead boat of the Vanguard class of submarines which carry Trident ballistic missiles around the world. Le Triomphant is also the lead ship in her own class of Triomphant nuclear submarines. Each carries 16 M45 ballistic missiles, weighs 35 tons each, carries six warheads and has a range of around 5,000 miles. France's Atlantic coast is notorious for being a "submarine graveyard" because of the number of underwater craft, mainly German U-boats, sunk in the area during the Second World War.