A Spanish warship threatened to open fire on American treasure hunters yesterday as they tried to flee Gibraltar in the battle for a haul of gold and silver coins estimated to be worth half a billion dollars.

Amid rough seas, The Times watched from aboard the Odyssey Explorer as Spanish Civil Guards made two unsuccessful attempts to board the ship minutes after it set sail. However, high waves and driving rain made the operation impossible, prompting them to force the ship to port instead.

“Under the threat of deadly force, we have had to turn the ship around and proceed to Algeciras,” Aladar Nesser, an official from Odyssey Marien Exploration, said as the ship was escorted to port. The company claimed the ship was in international waters outside Gibraltar when the Spanish cut her off with a Corvette warship, a police patrol boat and a high-powered dinghy. Spain claims those waters are within its territorial boundaries.

Spanish officers served notice that they were investigating Odyssey’s suspected plunder of underwater archaeological sites and confiscated millions of pounds worth of evidence, including computer hard drives and the company’s multimillion-dollar underwater robot. Sterling Vorus, the ship’s captain, was arrested for “grave disobedience” after refusing to let the police on board, and taken into custody.

The operation was the latest skirmish in an international fight over what could be the greatest underwater treasure yet discovered. The haul of 500,000 coins has pitted a pugnacious company owned by Bob Hope’s former public relations man against a Spanish Government enraged at what it views as modern-day piracy.

It has also set the archaeological community – which views Odyssey’s profit-driven scans of the ocean floor with suspicion – against commercial salvage firms, whose deep pockets and high-tech submersibles are making it possible to explore underwater wrecks like never before.

And it has reignited Spanish anger over ownership of the Rock of Gibraltar, the tiny British colony at the centre of a 300-year tug-of-war between two former maritime powers.

The fight over the mytery shipwreck began earlier this year when Odyssey secured the consent of the British and Spanish Governments to dig-up the HMS Sussex, an 80-gun warship lost off the coast of Gibraltar in 1694. Odyssey says the wreck may contain coins worth billions of dollars today. But instead of the Sussex, Odyssey announced in May that it had found a huge treasure in a different wreck at a site it codenamed Black Swan – and that it had already spirited it away to Tampa, Florida.

Odyssey says that it acted entirely legally and only after the Spanish Government refused to reach a deal over how to share any discoveries in area. It says that the wreck was in international waters, around 180 nautical miles west of Gibraltar, but it will not say exactly where, claiming that such information could lead to looting.

Spain suspects that the wreck at Black Swan is a Spanish galleon called Nuestra Señora de las Mercedes, sunk by the British fleet off Portugal in 1804. Court papers show that at least some of the coins recovered are Spanish, leading officials to believe that Odyssey recovered a Spanish treasure from a Spanish ship – even if it was outside its territorial waters.

Madrid’s US lawyer James Goold accuses Odyssey of engaging in an illegal game of hide-and-seek and has filed suit against the company in Tampa seeking the return of the treasure, as well as making claims on two other wrecks found by Odyssey.

Spain also accuses Odyssey of acting in a deceptive manner by playing down the value of its find until it had been spirited to the US. In its original export licenses filed with the Government of Gibraltar, Odyssey described its cargo as “encrusted clumps of silver coins in a corroded condition”, valuing the coins at $2.5 million, or $5 apiece. By June, however, the company said the coins had been valued at up to $500 million, or $1,000 each, sending the company’s shares sharply higher.

Nick Bruyer, and expert who examined a sample of the coins, described the haul as “unprecedented”. “I don’t know of anything equal or comparable to it,” he said.

Odyssey, meanwhile, has sued Spain for financial losses caused by the detention of its other boat, the Ocean Alert, in an almost identical manoeuvre in July. It is also seeking compensation for losses caused by the “obstruction of its operations” while its ships were stuck in Gibraltar .

Under the principle of “finders keepers”, admiralty courts tend to award the bulk a treasure to those who raise it from the deep. But it is not always so. Spain has dug in its heels and any legal battle could be protracted.

Meanwhile, Odyssey officials say that an undisclosed portion of the treasure still lies in the deep, protected only by its silence.

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