The Britsh Royal Navy's HMS Vanguard and the French Navy's Le Triomphant are both nuclear powered and were carrying nuke missiles when the collision took place on Feb 3-4.
The crash is believed to have occurred after state-of-the-art technology fitted in both vessels, which is designed to detect other submarines, apparently failed completely
"The potential consequences are unthinkable. It's very unlikely there would have been a nuclear explosion. But a radioactive leak was a possibility. Worse, we could have lost the crew and warheads. That would have been a national disaster," The Sun quoted a Navy source, as saying.
The collision is believed to have taken place on Feb 3 or 4, in mid-Atlantic. Both subs were submerged and on separate missions.
As inquiries began, naval sources said it was a millions-to-one unlucky chance both subs were in the same patch of sea. Warships have sonar gear, which locates submarines by sound waves.
But modern anti-sonar technology is so good it is possible neither boat "saw" the other.
A senior military source said: "The lines between London and Paris have been hot."
The MoD insisted last night there had been no nuclear security breach. But this is the biggest embarrassment to the Navy since Iran captured 15 sailors in 2007. The naval source said: "Crashing a nuclear submarine is as serious as it gets."
Vanguard is one of Britain's four V-Class subs forming our Trident nuclear deterrent. Each is armed with 16 ballistic missiles.&13;&13;