By BARRY WIGMORE

Last updated at 22:12 18 May 2007

It's a haul that would make pirate Jack Sparrow's eyes water.

Treasure hunters have discovered what they believe to be the world's most valuable shipwreck at a location thought to be off the Cornish coast.

Even as Captain Jack prepares to make his cinematic return in Pirates Of The Caribbean: At World's End, it is instead Land's End where the multi-million-pound treasure trove is suspected to be languishing.

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Using state-of-the-art mini-submarines, hunters have already recovered 17 tons of 17th-century gold and silver coins from the wreck - codenamed the Black Swan - worth at least £250million.

Experts said last night that there could be hundreds of millions of pounds worth of bullion still on the ocean floor.

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The treasure found so far includes 500,000 silver coins, hundreds of gold coins, gold ornaments and tableware and other golden artefacts.

The discovery was made by secretive U.S. underwater treasure hunt company Odyssey Marine Exploration.

Under international salvage law the company, of Tampa, Florida, could get up to 90 per cent of its find, depending on who the ship's original owners were and if any other claimants come forward.

Some of the bounty could go to the British Government, although Odyssey says the wreck is in international waters.

Fearing the descent of maritime salvage pirates - and with Penzance nearby, who can blame him? - company founder John Morris refused to reveal what wreck his team has found, or its location.

Odyssey has also filed excavation for two other sites: one 100 miles west of Gibraltar and one about 65 miles east of Sardinia.

But as these claims were filed only last month, it is not thought that there would have been enough time to mount such a complex expedition yet at either site.

That leaves the Land's End site, for which the claim was filed last September, as the likeliest location.

Experts in the U.S. were last night examining the 17-ton haul so far recovered from the Black Swan.

Rare coin expert Nick Bruyer, who was called in to examine part of the booty, said: "For this colonial era, I think the find is unprecedented. I don't know of anything equal or comparable to it."

Odyssey co-founder Greg Stemm said: "The remarkable condition of most of the first 6,000 silver coins conserved has been a pleasant surprise and the gold coins are almost all dazzling, mint-state specimens.

"We are excited by the wide range of dates, origins and varieties of the coins and we believe that the collecting community will be thrilled when they see the quality and diversity of the collection."

The silver coins from the wreck could be worth several thousand pounds each.

John Morris, Mr Stemm's partner in Odyssey, said: "Our research suggests that there were a number of colonial period shipwrecks lost in the area where this site is located, so we are being very cautious about speculating as to its possible identity."

He insisted that they were not just plundering the wreck, adding: "We have treated this site with kid gloves and the archaeological work done by our team out there is unsurpassed. We are thoroughly documenting and recording the site, which we believe will have immense historical significance."

Until now the richest shipwreck found was the Spanish galleon Nuestra Senora de Atocha, which sank in a hurricane off the Florida Keys in 1622.

Treasure-hunting pioneer Mel Fisher found it in 1985 and recovered £200million-worth of coins.

In 2004 Odyssey discovered the SS Republic, a shipwreck lost in 1865 off the U.S.coast. The company recovered more than 50,000 coins worth more than £40million.

Since then it has spent a fortune on searches, making a £2million loss for the first quarter of 2007.

It is currently on the look-out for an even bigger payday than the Black Swan - the British warship HMS Sussex, which sank in the Straits of Gibraltar in 1694.

The nine tons of gold bullion still thought to be inside would be worth around £2.5billion, most of which would go to the Treasury under a unique salvage agreement.