SANTIAGO DE COMPOSTELA, Spain - Communities along Spain's northern coast girded for widespread oil contamination after a crippled tanker holding twice the load lost aboard the Exxon Valdez split in two yesterday and sank about 125 miles offshore.

About 2 million gallons of its 20 million-gallon load from Russia spilled when the Prestige, a Greek-owned, Bahamian-registered tanker, foundered after leaking a trail of oil for six days from a widening crack in its unlined hull.

The sinking prompted calls from environmental groups for accelerating a shift to sturdier tanker designs with double hulls designed to cut risks of big spills. Deadlines set under an international agreement allow some single-hulled tankers to continue sailing until 2015 or later.

The sinking raised questions about the decision of Spanish officials to order the disabled 791-foot-long ship towed offshore, where its load, carried on the wind and tide, could now threaten a broader region.

Although birds and beaches were tainted in places from earlier leakage from the disabled ship, the scope of environmental damage from the Prestige remained relatively constrained yesterday. But drifting slicks could produce a major coastal disaster within a week, some European officials said.

European and American weather forecasters and oil-spill experts said winds out of the west were expected to build to 35 mph today and stay strong through Sunday, driving the slick toward dozens of coastal communities that depend almost entirely on tourism and fishing.

Officials said the ship's load was a particularly thick fuel oil that does not readily evaporate or break up.

"It's the kind that floats real low in the water, like the blobs in a lava lamp," said David M. Kennedy, the director of the office of response and restoration for the National Oceanic and Atmospheric Administration, which was monitoring the sinking but not directly involved in the cleanup.

It could take up to a week for the oil released yesterday to reach shore, European and American oil-spill officials said. Kennedy said the offshore slick was a kind of time bomb, but no one could yet know when it would go off.

Another looming question was what would become of the 90 percent of the load that apparently sank more than two miles to the seabed in compartments in the ship's sundered bow and stern sections.

Some oil continued to surface at the wreck site yesterday.

But chill temperatures at that depth, just a few degrees above the freezing point of water, were likely to keep the oil in a thick, almost waxy, state, some experts said.

Even in that congealed state, however, the oil was still more buoyant than water and could rise to the warmer surface and liquefy if the compartments shattered when the two parts of the ship hit the bottom.

"If the hull is broken by the shock when the ship hit the sea bottom, it will have a large failure, and through this, the oil can come up to the surface whatever its state - whether it's solid, pasty or liquid," said Cabioc'h Fanch, an engineer who investigates oil spills for Cedre, a French pollution-research group.

For now, there is no way to know whether that happened, he and other officials and experts said.

Some tankers have sunk in great depths without disgorging their loads, including one that was examined in deep water off Japan and found to have most of its compartments intact.

The Prestige, 26 years old, had carried its load of Russian fuel oil from a Latvian port in the Baltic Sea en route to Singapore when it encountered a storm last Wednesday off the coast of Spain and the hull began to give way.

Lars Walder, a spokesman for Smit Salvage, a Dutch company that had been hired to tow the ship to deep water and keep it afloat, said that the clear task a week ago was to prevent the damaged ship from running aground.

But then came the problems, he said.

"What we would have preferred at that time was to look for shelter in a bay and there pump out the oil into another ship," he said.

"Now," he said, "because we were ordered by the Spanish authorities to go into deeper water, the oil was spread over a larger, enormous amount of shore."