Spain demanded the return of sunken treasure worth an estimated half a billion dollars yesterday, accusing Odyssey, the deep-sea exploration company that discovered it, of looting its shipwrecks.

Spanish archaeologists said that they had determined “with complete certainty” that the record haul had come from the Spanish colonial-era galleon Nuestra Señora de las Mercedes, sunk by a British fleet off the southern coast of Portugal in 1804.

“The mystery is over,” said James Goold, a Washington-based lawyer for the Spanish Government. The treasure “belongs to the Spanish Armada”. Since announcing in May 2007 that it had found 500,000 gold and silver coins somewhere “in international waters in the Atlantic Ocean”, Odyssey has fought hard to keep details of the haul under wraps.

The Florida-based company, which uses underwater robots to search for shipwrecks, had refused to divulge the exact location of the find that it codenamed “Black Swan”. It also argued that because the haul included Spanish coins, it did not necessarily mean it had come from a Spanish ship.

However, Mark Pizzo, a US judge, ruled against Odyssey last month, ordering it to share information about the find with the Spanish. Yesterday Spain said that coins from the haul were documented to have been on the Mercedes, while the location of the find also indicated that it came from the galleon.

“The sinking of the Mercedes was a pivotal event in Spanish and European history, and the site and its contents are the inalienable historical heritage and patrimony of Spain,” the Government said in court papers due to be filed today as part of a continuing trial to determine who owns the treasure. It added that it never authorised Odyssey to disturb the “gravesite of hundreds of Spanish sailors and their family members” who died when the ship sank.

The haul has been at the centre of a fierce international fight between the Spanish Government and the small, publicly listed company, run by a flamboyant former public relations executive who once served as Bob Hope’s fixer. The Spanish Government has accused Odyssey of modern-day piracy and plundering its archaeological sites for profit.

Odyssey has responded that such sites would never have been explored by the Spanish Government, which had effectively abandoned the shipwreck. It claims that under the “finders’ keepers” provision of international maritime law, it has the right to the treasure.

Yesterday, however, Spain said that it was irrelevant that the ship sank in international waters. “We do not lose ownership by being in international waters if we’re talking about a warship,” Mr Goold said. “The 14 tons of coins were not transported on a commercial ship.” Spain’s lawyer accused Odyssey of engaging in obstructionism and behaving in an “illegal and immoral” manner. Odyssey had no immediate comment on the allegations by the Spanish.

The battle over what could be the largest treasure trove yet found turned particularly ugly last year, when the Spanish Government forced Odyssey’s two exploration ships into port, using warships as they tried to leave Gibraltar. In October Spanish police arrested the Odyssey Explorer’s captain, Sterling Vorus, for disobeying orders. He was released the following day.

Odyssey has accused the Spanish Government of bullying its staff and is claiming damages for losses relating to the case.

Spanish experts said yesterday that the coins, held at an undisclosed location in Florida, included gold doubloons, or “pieces of eight”, minted in 1803 in Peru — then the financial centre of Spain’s Latin American colonies. That raises the possibility that the Peruvian state may want to make a claim of its own.

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