A hoard of silver coins worth £34million that was sunk by the Nazis on board a steamship has been salvaged by a British-led team at a record depth of 5,150m (17,000ft).

The SS City of Cairo was travelling from Bombay to England in 1942 when it was torpedoed by a U-boat 480 miles south of St Helena, taking 100 tons of rupees with it to the bottom of the ocean.

It was long assumed that the vessel's cargo, belonging to the UK Treasury, would be lost forever such was the complexity of the task facing salvage experts.

Treasure trove: Some of the 100 tons of rupees found buried in the ocean floor at a record depth of 17,000ft after the cargo ship they were being transported on was sunk by a Nazi submarine during World War Two

Record find: Some coins are raised to the surface during a pioneering recovery project by a British-led team

Hidden: The City of Cairo was finally discovered (above) 'broken in two and buried deep in the seafloor silt'

Underwater company Deep Ocean Search used powerful sonar to locate the vessel on the sea floor

That was until a team led by British expert John Kingsford used pioneering techniques to track down the ship with powerful sonar and robotics.

Underwater company Deep Ocean Search (DOS), which included 20 French oceanographers, was then contracted by the UK Ministry of Transport to recover the coins from a record depth of 17,000ft - some 4,500ft deeper than the Titanic.

A spokesman for DOS said: 'This was to be a difficult search as the water depth would exceed 5,000m, the weather, swell and currents were known to be challenging and the presumed site was some 1,000 miles from the nearest land in the foothills of the mid-Atlantic ridge.'

When the ship was finally discovered, it was 'broken in two and buried deep in the seafloor silt', adding that 'parts of the ship had metres of mud heaped upon it.'

The SS City of Cairo was sunk en route from Bombay to India, via Cape Town and Recife in Brazil

The rupees, belonging to the UK Treasury, were being transported from India to England when the ship sunk

Underwater company Deep Ocean Search, which included 20 French oceanographers, was contracted by the UK Ministry of Transport to recover the coins from a record depth of 17,000ft - 4,500ft deeper than the Titanic

Keeping it under wraps: The recovery took place in 2013, but has only now been made public

'SORRY FOR SINKING YOU': U-BOAT CAPTAIN'S MESSAGE TO SS CAIRO The SS City of Cairo was travelling from Bombay to England, via Cape Town and Recife, Brazil, when it was attacked by a Nazi U-Boat on November 6, 1942. The U-68 hit the vessel with one torpedo, but waited a further 20 minutes before firing the second. That crucial delay gave 296 of the ship's 302 passengers and crew time to escape in lifeboats. Famously, the German captain, Karl-Friedrich Merten, approached the lifeboats and told the survivors: 'Goodnight, sorry for sinking you.' The victims were eventually rescued three weeks later, but 104 had died. Advertisement

Maeva Onda, one of the oceanographers on board the salvage ship, SV John Lethbridge, said the search submarine was successful on its first dive.

'After two hours of underwater descent, the robot transmitted the first images of the wreck. It was incredible,' she told Le Figaro, as cited by the Daily Telegraph.

The find was confirmed when the robot brought up coins stamped with the Crown.

But the team suffered 'serious' problems trying to recover the bulk of the hoard because of the depths involved.

The spokesman for DOS said: 'The team quickly found that operating at this depth caused serious technical difficulties which were new to us and which had to be resolved, quickly.

'The combination of pressure, temperature, repeated dives at this depth and other issues resulted in multiple breakdowns of systems such as we had not experienced before when working in 3000m to 4000m depths.'

The recovery took place in 2013, but has only now been made public.

DOS has left a plaque commemorating their find on the sea bed.