A SECRET mission to inspect Cold War nuclear submarines was the real reason the wreck of the ill-fated Southampton liner Titanic was discovered.

Oceanographer Bob Ballard and his team of underwater explorers were told they could only search for the liner once they found USS Thresher and USS Scorpion.

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Both US Navy submarines had sunk with the loss of more than 200 men during the 1960s amid suspicions that at least one had been targeted by the Soviet Union.

Dr Ballard was made head of two 1984 expeditions to find Thresher off the eastern coast of America and Scorpion in the eastern Atlantic.

Now he is able to talk for the first time about the missions as the files have been declassified by the US government.

He said: "I couldn't tell anybody. There was a lot of pressure on me. It was a secret mission. I felt it was a fair exchange for getting a chance to look for Titanic. We handed the data to the experts. They never told us what they concluded - our job was to collect the data."

Only once he had used his underwater robot craft to map the submarine sites was he allowed to use it to criss-cross the north Atlantic seabed and pinpoint Titanic in 1985.

This left a race against time with only 12 days to find the ship, which sunk after hitting an iceberg on her maiden transatlantic voyage from Southampton to New York in April 1912.

A total of 1,522 people died, including more than 500 from Southampton.

The US military was not willing to fund the search for Titanic when Dr Ballard first made an approach in 1982 but officials were desperate to know how their submarines had sunk and whether the nuclear reactors had been affected by being submerged for so long.

A favourable outcome would mean the US Navy might be able to dispose of unwanted reactors at sea.

The oceanographer found there was little risk to the environment from radioactivity and Scorpion, which disappeared in 1968 with 99 crew, had most likely been hit by a rogue torpedo it had fired itself.

Thresher, the US Navy's most advanced attack submarine at the time, had sunk in 1963 with 129 crew after a high-pressure cooling pipe supplying the nuclear reactor had burst.

The accident, 1,000ft beneath the surface, meant the vessel lost power, sunk deeper and imploded into thousands of pieces.

It was while tracking the two vessels that Dr Ballard hit upon the idea of hunting for Titanic by searching for a trail of wreckage.

"It was as though it had been put through a shredding machine. There was a long debris trail."

n The story will be told in a TV documentary called Titanic: The Final Secret, which will be shown on the National Geographic Channel at 9pm on June 8.