The interception of the treasure-hunt ship off the coast of Gibraltar is the latest broadside in a tense battle between a US-based salvage company and the Kingdom of Spain over an unidentified shipwreck and the ownership of its rich haul of gold and silver coins.

On Tuesday, patrol boats from Spain’s maritime police intercepted the 76m Odyssey Explorer, owned by underwater salvage firm Odyssey Marine International, three miles off the coast of Gibraltar. It was escorted to the Spanish port of Algeciras.

Spain’s Civil Guard has been keeping a close eye on the company’s vessel since a Spanish judge ordered that it be detained and searched if it left port in Gibraltar.

The company says its recovery vessel has been effectively blockaded since the ruling in June. Spain believes it could provide clues to the identity and location of the wreck that yielded half-a-million colonial era silver and gold coins.

It suspects that a Spanish galleon is being secretly plundered - or that the wreck lies in Spanish waters.

Odyssey Marine Explorations - which became the most famous deep water treasure hunting company when it announced the discovery last May - says it is keeping the location of the wreck secret, to protect the site from looters.

All it is saying is that the wreck - codenamed Black Swan - is somewhere in the Atlantic.