The families of the five crew who died on the Bugaled Breizh in 2004, are hoping a coroner's inquest in London will confirm their view that the boat was pulled down by the British sub HMS Turbulent.

But the head of submarine operations for the Royal Navy, Commander Daniel Simmonds, told the hearing in London that the vessel was at Devonport docks in Plymouth, southwest England, at the time of the tragedy.

The nuclear attack submarine was due to take part in NATO exercises in the area from January 16, 2004 -- the day after the tragedy.

Simmonds said it was unable to do so due to damage, maintaining it was "unthinkable" that the logs would be falsified.

The ramifications of such a move "would be significant" and would "erode trust" between allies, he told a judge at the High Court in London.

He also said it was "unthinkable" that an allied submarine from another country taking part in the exercises could have been in the area without signalling its presence.

From the outset, the families have said they believed that the Breton trawler sank after a submarine caught its nets and dragged it down.

British naval exercises were scheduled for the day of the tragedy, but without submarines, according to Simmonds.

He told the court that only three NATO submarines were at sea when the Bugaled Breizh sank; the German U22, the Dutch Dolfijn and the British Torbay.

He said it was impossible that any of them could have been within five nautical miles of the trawler.

When questioned on Monday, the Dutch Navy ruled out any involvement of the Dolfijn submarine, saying it was on the surface when the accident occurred.

A long legal procedure in France ended in 2016 with no decision reached on whether a submarine caused the tragedy or if it was a fishing accident.

The British court is due to hear from Andrew Coles, the former commander of the Turbulent later Tuesday.