An American treasure-hunting firm, which gained notoriety after it was ordered to hand over £308m of coins it salvaged to the Spanish government, has recovered a haul of silver worth a fortune from a British merchant ship sunk by a German U-boat during the second world war.

The 48.8-tonne haul of silver recovered off the coast of Ireland by the salvage company is estimated to be worth about £24m.

Odyssey specialises in deep-ocean exploration. The bullion was found three miles under the Irish sea in the wreckage of the Gairsoppa, a 412-foot steel-hulled cargo ship torpedoed by the Nazis 70 years ago.

The company, based in Tampa, Florida, believes the silver recovered so far is just a fifth of what was on board when the ship went down. If this proves to be the case, the value of the whole cargo could be £121m.

Odyssey said that under an agreement with the British government, it is allowed to keep 80% of the treasure after recouping its expenses. It added that the Gairsoppa silver had been moved to “a secure facility” in the UK.

Earlier this year, leading British archaeologists criticised the government for letting the company excavate the 18th-century warship HMS Victory laden with a potentially lucrative cargo, including gold and silver worth many millions of pounds.

A similar salvage operation five years ago turned into a storm for Odyssey after Spain claimed ownership of £308m of gold and silver coins found in the wreck of a Spanish frigate off the Portuguese coast, which the company called “Black Swan”. To the fury of Spanish authorities, the firm landed the trove in Gibraltar and flew it out in chartered aircraft to its base in Florida.

The epic battle over ownership of the treasure ended with victory for the Spanish government, with US and Spanish judges ordering Odyssey to return the haul.

The dispute then moved to the US courts, where earlier this year a judge ruled in favour of the Spanish government, forcing the company to hand over the gold and silver bullion.

Odyssey is also involved in a project to recover silver from the SS Mantola, which was sunk by a German submarine in February 1917 about 100 miles from the Gairsoppa site.