A mystery surrounding the deaths of five French fishermen in a trawler accident three years ago deepened yesterday when a judge said that the boat was probably sunk by an unknown submarine spying on Nato exercises.

Richard Foltzer, a French investigating magistrate, made his ruling after a campaign by relatives of the dead men to break through what they describe as a wall of silence put up by France, Britain and other Nato members.

They have always believed that a submarine caused the sinking of the Breton trawler Bugaled Breizh off Cornwall in January 2004, and that officials have covered up the truth.

They claimed to have been vindicated when Judge Foltzer told them that the most plausible theory was that one of the trawler’s cables had been caught by a submarine which then dragged it under water.

“The legal system first talked about a submarine nautical event, then an exogenous force and now they are pronouncing the word submarine,” said Maître Christian Bergot, the lawyer representing the fishermen’s families. “The judge has made striking progress.”

Judge Foltzer appears to have ruled out claims that the accident was caused by a British or Dutch submarine participating in the joint Nato and Royal Navy exercise.

Documents submitted by the British authorities say that HMS Turbulent – accused in a French television documentary of involvement in the sinking – was tied up in Devonport while HMS Torbay was 100 miles away. The Dutch vessel, Dolfinj, was nine miles away.

After meeting Judge Foltzer, Maître Bergot said: “The submarine expert designated by the judge has put Nato’s submarines out of the area.

“If it’s not one of those, it can only be a spy submarine,” he said the judge told him.

He welcomed judicial backing for what relatives claim to be the most likely explanation for the loss, but said that it would be very difficult to trace the submarine allegedly responsible.

Rémi Gloagen, whose brother Patrick died in the sinking, said: “We were happy that the word ‘submarine’ was pronounced but when the spy submarine was mentioned we said to ourselves, ‘We’re at a dead end’.”

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