The Spanish government's representative in the Galicia region, Arsenio Fernandez de Mesa, said he was arrested on suspicion of disobeying authorities and harming the environment.

Local newspaper La Voz de Galicia said Mangouras refused for several hours on Wednesday night to allow tugs to tow his listing ship away from the Spanish coast while he haggled over the price of the tugs.

The 26-year-old "Prestige" got into difficulties amid a violent storm on Wednesday off the scenic and unspoiled stretch of Galician coastline known as the "Coast of Death".

One of its tanks was holed due to unknown causes and around 5,000 tonnes of oil flowed out, creating a long oil slick that is now some seven miles (11 km) off the Galician coast, an important fishing area with a rich variety of birds.

But some 72,000 tonnes of fuel oil remain aboard the tanker, posing the risk of environmental disaster.

Environmental group WWF said that if all the fuel oil leaked, it would be one of the largest oil spills in the world, about twice as big as the Exxon Valdez spill off Alaska in 1989.

WWF said the first oil-covered birds were reaching the coast. It highlighted the risk to the coast and to an environmentally rich marine area 200 km (124 miles) off the Galician coast if the tanker sinks.

Tugs towed the single-hulled tanker away from the Spanish coast during the night but a hole in the tanker widened.

"Inspectors...are telling us that the ship runs the risk of breaking up," Fernandez de Mesa said. Later, he told reporters the threat of a breakup was not imminent.

OBSTACLES TO SALVAGE OPERATION

The weather deteriorated later last week, with waves up to eight metres (yards) high. The tanker was still being towed and was about 95 miles (153 km) off the Spanish coast, said Lars Walder, a spokesman for Smit Salvage, a Dutch company contracted to try to salvage the "Prestige".

Smit Salvage was looking at the possibility of patching the tanker or transferring its cargo to another vessel, he told Reuters from Rotterdam.

But he said: "At the moment, it's very difficult for us to work. We cannot do any ship-to-ship transfer of the oil because of bad weather conditions."

The "Prestige" has left in its wake a bubbling diplomatic row between Madrid and London over the British colony of Gibraltar on Spain's southern coast.

Spain alleged that the "Prestige" regularly plied the route between Latvia and Gibraltar, neither of which, it said, belonged to the international body responsible for ship inspections. It says the ship was last inspected in 1999.

Crown Resources, which owns the oil on the "Prestige", said the vessel had been headed for Singapore, contradicting earlier reports its destination was Gibraltar.

Deputy Prime Minister Mariano Rajoy said Spain would lodge protests with several countries over the state of the ship and an alleged failure to take adequate safety measures, including Britain over safety standards in the port of Gibraltar.

The British ambassador to Madrid, Peter Torry, said in a statement that suggestions the "Prestige" was linked with Gibraltar were "complete nonsense".