The Bahamas-flagged Prestige vanished into the ocean at midday, said Lars Walder, a spokesman for the Dutch salvage company SMIT. The ship's oil containers seemed to remain intact, but the toxic fuel was likely to seep out eventually.

An environmentalist warned the wreckage would be like a "time bomb" about two miles down on the ocean floor. Nearly 1.3 million to 2.6 million gallons of fuel oil lost in the initial spill last week have already tainted miles of Spanish beaches, threatening rich fishing grounds and devastating wildlife.

If the ship lost its entire cargo of fuel oil, the spill would be nearly twice the size of the 1989 Exxon Valdez disaster in Alaska. Some 10.92 million gallons of crude oil were lost from the Valdez.

The tanker ruptured last Wednesday during a storm, and was towed some 150 miles out to sea. The crew was airlifted to safety last week.

The incident caused friction between Portugal and Spain, which disagreed over who was responsible for the cleanup. Prevailing winds put Spain's coast at a greater risk for damage from the spill. It also irritated relations between Britain and Spain amid charges that port inspectors should have spotted obvious problems with the ship, which had stopped repeatedly in the British colony of Gibraltar. The two countries have long been at odds over ownership of the rocky promontory on Spain's southern coast.

Spanish beaches were mired in oil and scores of animals were covered in sludge. Fishing was prohibited, putting hundreds out of work. The spill threatened some of the region's richest fishing grounds.

Fuel oil, a heavy, viscous blend gathered from the bottom of tanks at the end of the refining process, can be far more toxic and difficult to clean up than crude oil, experts said.

"We hope that the sunken part does not spill its fuel. But still it's a time bomb at the bottom of the sea," said Maria Jose Caballero, who leads the coastal protection project for Greenpeace in Spain.

The best hope for the environment is for the tanks to hold in the chilly waters, said Unni Einemo, senior editor at Bunkerworld, a London-based news service for the marine fuels industry.

"If it sinks into cold water, this stuff solidifies so much that it basically stays there," she said.

The Prestige, which flies under a Bahamian flag, is owned by Mare Shipping Inc., which is registered in Liberia, and managed by a Greek company, Universe Maritime, Ltd. It was traveling from a Baltic port to Singapore, carrying a load of Russian oil, when the storm hit, Universe Maritime said.

The American Bureau of Shipping, a Houston-based registration company that makes sure shipping papers are in order, said the Prestige was up to date with its inspections.

The vessel, built in 1976, had its last annual survey in Dubai in May, and a full drydock inspection in China in May 2001, ABS said.

A Universe Maritime spokesman complained that the damaged vessel had been exposed to storms because it had been forced so far off shore. The Spanish government had ordered the ship far from land to limit contamination.

The tanker sustained a 30- to 50-foot crack in its hull below the waterline which made it unable to proceed under its own power while salvagers sought a port to do repairs or transfer the oil to another ship.

The Oil Spill Response Center in Southampton, England, a nonprofit organization owned by the international oil industry, sent a team to help Spanish officials deal with the spill. Detergents are ineffective for cleaning up fuel oil, he said.

"It is the most difficult of the fuels to clean up," said the group's chief executive, Archie Smith, adding that the toxicity of fuel oil is diminished somewhat after it comes into contact with sea water.

Spanish soldiers and volunteers were using buckets and shovels to remove oil some 40 miles of coastline between Cape Finisterre and the city of A Coruna, about 370 miles northwest of Madrid. Elsewhere, emergency workers tried to vacuum oil from beaches.

Sea birds floated helplessly in the blackened waves and fish washed ashore. Volunteers tried to rescue about 150 of the injured animals.

"We've seen many dead fish and birds and many others in agony when we rescue them," said Ezequiel Navio, from the World Wildlife Fund's Spanish branch.

Spain's Interior Ministry said the ship went down in an area where Portugal had responsibility for maritime rescues. Portuguese Prime Minister Jose Durao Barroso said it was "absolutely sure and confirmed" by the Portuguese Navy that the tanker was lying in Spanish waters.

Both Portugal and Spain had barred the salvagers from towing the ship to any of their ports to protect their fishing and tourism industries.

The tanker's Greek captain, Apostolus Maguras, was jailed on charges of disobeying authorities and harming the environment.

In Brussels, EU officials demanded governments move faster to enforce new inspection rules that could prevent such catastrophes.

Under the rules, ports are required to check at least 25 percent of all ships coming in, starting with older, single-hull vessels. Ships flying "flags of convenience" -- or registered in countries with lax safety, labor or tax rules -- are to be given priority, said Gilles Gantelet, spokesman for the European Commission.

The regulations are meant to catch ships like the Prestige, which stopped in Gibraltar last June. Authorities there say it did not enter the port, but only stopped for restocking. It also put in on June 7 at the Greek port of Kalamata, but Greek officials said because it was "in transit," it was not subject to inspections under treaty obligations.

EU officials attacked such distinctions as potentially dangerous.

The EU measures were adopted after the tanker Erika broke apart and sank in 1999, discharging 3 million gallons of fuel oil and polluting 250 miles of French coastline.

Spain's northwest coast has suffered several tanker accidents in recent years. The worst was in 1992, when the Greek tanker Aegean Sea lost 21.5 million gallons of crude oil when it ran aground near A Coruna.