Kostas Thoctarides told ANA his team had located the wreck of HMS Triumph at a depth of 203 metres (666 feet) at an undisclosed location in the Aegean Sea.

The search for the submarine began in 1998 and was "the hardest and most expensive mission I have ever carried out in my life", Thoctarides told the agency.

Research in archives in the UK, Germany, Italy and Greece helped to unlock the submarine's final resting place, he added.

The 84-metre T-class submarine was launched in 1938 and carried out twenty missions, including attacks against Axis ships, landing British commandos and rescuing Allied soldiers.

Thoctarides -- who in 1997 located the hulk of HMS Perseus, a British submarine that sank in 1941 -- said the Triumph was last sighted by an Italian pilot near Cape Sounion near Athens.

The last attack by the vessel is believed to have been against an Axis cement freighter.

It was officially declared missing with 64 people on board in January 1942.

Thoctarides told ANA that the submarine's periscopes and hatches were down, indicating that it was in a deep dive during its final moments.

It appears to have sunk due to a powerful explosion in the fore section but the cause of the blast is still unclear, he said.