The ship was listing heavily and situated 4.6 nautical miles from the shore when towing operations began, Spain's Development Ministry said in a statement.

The spill of an estimated 5,000 tonnes of oil threatened marine life but officials were concentrating on preventing the tanker from breaking up and causing a much bigger environmental disaster.

The ship ran into trouble in a force eight gale on Wednesday 30 miles (50 km) off Cape Finisterre, the most westerly part of Galicia, a region famed for its seafood and unspoiled beaches.

Twenty-four crew members were airlifted to safety, but the captain and two other officers remained aboard the 26-year-old tanker to help salvage efforts.

Jose Luis Lopez, the local maritime rescue director, told Spanish radio that two of the ship's tanks spilled an estimated 5,000 tonnes of fuel near the so-called "Coast of Death".

Authorities said a slick measuring 200 metres by 20 miles stretched in the wake of the Prestige, which was blown toward the coast by strong winds overnight. The wind changed last week morning and was blowing from the southwest, which helped keep the tanker from foundering on the rocky coast.

"Four crew members plus one of our technicians are now on board trying to restart the engines," a rescue services spokesman in Galicia said earlier.

There would be an investigation into why the crew had decided to stop the tankers' engines, he said, but added there had been no fire in the engine room, as one report had said.

Spain's Development Ministry could not immediately confirm radio reports that the ship's motors had been restarted.

OLD, SINGLE-HULLED TANKER

A spokesman for the Greek managers of the "Prestige" said it was a single-hulled vessel.

The European Union passed legislation to ban single-hull oil tankers from its waters by 2015 after the tanker "Erika" ran aground off northern France in December 1999, spilling 15,000 tonnes of heavy fuel oil.

A spill from the "Prestige" could prove particularly damaging because fuel oil is heavier and denser than crude, making a slick less likely to break up in the sea and more difficult to clean up, although one official said cold water may help the oil sink quickly to the bottom.

The "Prestige" had sailed from the Baltic port of Ventspils, in Latvia heading for the British colony of Gibraltar.

Spain's Development Ministry said it had asked the Foreign Ministry to protest over the incident to Britain, the Bahamas, Latvia and Greece, as the ship has Greek managers.

The ministry said Latvia and Gibraltar did not belong to the Paris Memorandum of Understanding, a body responsible for ship inspections. It said the tanker, built in Japan in 1976, was last inspected in 1999.

The vessel, owned by Mare S.I. of Liberia and chartered by Russian trading house Crown, is among the oldest tankers still at sea. Major oil companies have a self-imposed ban on chartering tankers that are more than 15 years old.

The accident was the most serious in Spanish waters since a Greek tanker, the "Aegean Sea", ran aground in stormy weather near the north coast port of La Coruna almost exactly 10 years ago. The ship broke in two and caught fire, causing an oil slick that contaminated large tracts of coastline. (Additional reporting by Stefano Ambrogi and Sujata Rao in London, Adrian Croft, Blanca Rodriguez, Julia Hayley and Daniel Flynn in Madrid).