France's Le Triomphant, a nuclear missile submarine, seen in this undated photo released by the French Navy on Monday Feb. 16, 2009.

The HMS Vanguard was towed back to a base in Scotland with visible dents and scrapes. Le Triompant suffered damaged to a sonar dome, but returned under its own power to its base on L'lle Longue on France's western tip.

French Defense Minister Herve Morin denies the submarines had been shadowing each other. He says it was a freak accident, claiming the vessels make less noise than a shrimp.

A UK retired navy officer and submarine expert says there are a number of technical issues to be investigated, and the root of the problem appears to be procedural.

It's estimated repairs will cost around 70 million US dollars.

France and Britain may consider coordinating underwater patrol to avoid future collisions.

(CCTV February 19, 2009)