Odyssey says the ship was seized in international waters

The guard had been ordered by a Spanish judge to seize the vessel as soon as it left the British colony of Gibraltar.

Gibraltar officials and Odyssey Marine Exploration, which owns the ship, said Spain had boarded the ship illegally as it was in international waters.

In May, Odyssey said it had found $500m (£253m) in coins from a 17th Century wreck somewhere in the Atlantic Ocean.

Madrid suspects the sunken galleon may either have been Spanish or have gone down in Spanish waters.

The salvaged booty, which included half a million silver coins and hundreds of gold objects, has already been flown back to the US.

'Threat of force'

After leaving Gibraltar, the Ocean Alert was picked up at about 0700 GMT on Thursday off Europa Point and sent to the Spanish port of Algeciras to be searched, the Civil Guard said.

RULES OF THE WAVES Seas and oceans governed by UN Laws of the Sea Ownership of sunken property in international waters governed by the law of salvage and the law of finds Law of salvage: If property is owned, those finding it are entitled to compensation for their salvage efforts Law of finds: Salvor is entitled to all reclaimed property if it is proved to be abandoned Spain insists it retains rights to all its sunken treasure Odyssey intends its haul to be dealt with under US federal law, where previous judgements have sometimes granted exclusive rights to salvors

The guard was investigating a possible "offence against Spanish historic heritage", it said in a statement.

Odyssey said the boarding was illegal and said the Civil Guard threatened to use force if Ocean Alert's captain did not follow orders. It said Spain had earlier promised the ship would be searched at sea.

"At this point, Odyssey is assuming that the action on the part of the Guardia Civil is a miscommunication between Spanish authorities," the Florida-based company said in a statement.

A spokesman for the governor of Gibraltar said the ship was in international waters at the time it was seized.

A lawyer for Odyssey, Allen von Spiegelfeld, told Reuters news agency that Spain had not sought permission to board Ocean Alert from officials in Panama, where it is registered.

"The owners of the vessel have contacted the Panamanian maritime authorities protesting the seizure on international waters," Mr von Spiegelfeld said.

Spain's Civil Guard is searching the Panama-registered ship

Some experts believe the wreck to be the Merchant Royal, an English ship carrying stolen Spanish treasure which sank in 1641.

US coin expert Dr Lane Brunner has said there is evidence the shipwreck was found off England's Cornish coast.

Odyssey has kept the location of its find secret, citing security and legal reasons.