A CORUÑA, Spain, Nov. 19 — Communities along Spain's northern coast girded for widespread oil contamination in the coming days after a crippled tanker holding twice the load lost aboard the Exxon Valdez split in two today and sank more than 100 miles offshore.

About 2 million gallons of its nearly 20-million-gallon load 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 today. 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 up to 35 miles per hour on Wednesday 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, 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 today to reach shore, European and American oil-spill officials said. Some meteorologists said that intensifying winds, combined with south-flowing currents, could carry some of the oil south toward Portugal, as well as east along Spain's Galician coast. 

Another looming question was what would become of the 90 percent of the load that apparently sank more than two miles to the sea bed in compartments in the ship's sundered bow and stern sections. Some oil continued to rise the surface at the wreck site today.

But low 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 again 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 sea 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. Sonar surveys or other techniques might eventually determine the condition of the ship.

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

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

There have been several substantial oil spills in the region in recent decades. France only recently completed a cleanup from a spill in 1999, when the Erika, a similar tanker, also with an unlined hull, split in two in the Bay of Biscay and sank.

Fisheries have tended to recover quickly, officials said, although in several Spanish ports already affected by the early leakage from the Prestige, boats remained at their docks under a temporary fishing ban.

The Prestige's hull steadily weakened as cracks amidships grew while the ship was being towed away from shore. A lengthening trail of slowly leaking oil followed the ship for more than 100 miles. Finally, in death throes captured on Spanish television, the stern sank this morning and the bow about four hours later. 

The rupture amidships appeared to breach three or four of the tanker's internal compartments, releasing 6,000 to 8,000 metric tons, or 1.5 million to 2 million gallons, of oil, according to 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.

He and others involved in trying to save the ship or prevent spillage criticized some of the decisions made by government officials as the incident unfolded.

Mr. Walder said that the clear task a week ago was to prevent the damaged ship from running aground as it approached the Spanish coast. Then came the problems. "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. "Then you also have pollution but that's more or less controlled because it's more or less only in a small area."

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

Spain blames the British colony of Gibraltar for the crisis, saying the tanker was headed there. It has also clashed with Portugal, with both countries insisting that the other now take responsibility for the Prestige.

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