The boat sank off the coast of Norway more than 20 years ago with the loss of all its crew.

Sir Ray Halliday: Trawlers were used for spying

Relatives of the 36 men who died have always disputed the official line that it was destroyed by the sea.

They say it was on a government spying mission.

Some of the relatives hope the first survey of the wreck site will solve the mystery of how the ship was lost.

Trawlers were on the lookout for Russian submarines

The loss of the Gaul on 8 February 1974 is one of the most enduring seafaring mysteries.

The state-of-the-art supertrawler sank without even sending out a distress call.

Within days of the tragedy rumours began to circulate that the Gaul was the victim of the Cold War and paid the ultimate price for spying on the Soviet fleet.

For more than 20 years the government denied that any British trawler was involved in intelligence gathering.

Rir Ray Holliday: Trawlers could photograph Soviet naval movements

But speaking publicly for the first time, the Director of Naval Intelligence at the time of the Gaul tragedy says the use of trawlers for spying was widespread.

Vice-Admiral Sir Ray Halliday said: "Their fishing ventures took them into areas where the Soviet navy would be passing through, they could observe, photograph them and pass to us their course and speed so that some higher level of intelligence could get involved."

The former skipper of another Hull trawler remembers how he was recruited by MI6.

Mason Redfearn: He took pictures of Soviet navy vessels

Mason Redfearn said: "As soon as we saw Russian ships I would get the camera ready."

In 1974 a public inquiry ruled that the Gaul foundered in heavy seas.

A year later another inquiry reached a different conclusion - that a hatch door may have been left open.

Nearly 25 years on, the relatives of those who died are anxiously waiting the results of the new investigation.

The government still denies the Gaul was a spy ship, but relatives do not know what to believe.

Bery Betts: Lies and contradictions

Victim's sister Beryl Betts said: "We have been told so many lies and contradictions over the years.

"I will never accept it went down in heavy seas."

Victim's father Roy Wilson said: "I will never give up hope that he is alive somewhere in Siberia."

Victim's son Ken Collier said: "Our main objective is to see if the bodies are still on the ship.

"Secondly, we want to know how it sunk."

The three-day exploration could prompt a salvage operation as well as add to pressure for a reopening of the public inquiry.

But whatever the survey's findings, they are likely to fuel rather than conclude the Gaul controversy.