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Deputy Prime Minister Mariano Rajoy said the submarine would be able to see if the tanker's cargo had solidified in the chilly depths of the Atlantic as some experts hope.

The Liberian tanker broke up and sank on Tuesday with 77,000 tons of oil on board, and is now lying at at a depth of 3,500 metres off Spain's northern coast.

Oil washed ashore has left thousands of fishermen out of work, contamined more than 100 beaches and caused untold damage to the environment.

A satellite image shows the slick spreading from the wreck (l) to the coast on 17 November

Estimates of the amount of oil that has already leaked from the tanker vary from 10,000 to 20,000 tons.

Not all experts agree that the oil left in the tanker has solidified. Some fear that the Prestige's tanks may have ruptured when it hit the sea bed and that oil is still pouring out.

In 1989, when the Exxon Valdez oil tanker ran aground near Alaska, 37,000 tonnes of oil was spilled, causing the worst environmental disaster of its kind to date.

	:	Sinking tanker
Pictures of the Prestige oil slick

A major slick of some 3,000 tonnes has already hit the Spanish coast, in a region dependent on fishing and tourism.

	'Prestige' Built: 1976 Weight: 42,000 tons Cargo: 77,000 tons of oil Owners: Mare Shipping Registered: Bahamas Vulnerable vessel

A fishing ban along long stretches of the coast was extended on Friday.

Mr Rajoy said four other slicks were close to the coast, and were being driven towards the shore by high winds and big waves.

He outlined a recovery plan including 200 million euros of cheap or interest free loans, tax breaks and easier access to welfare.

Apart from the French submarine, a fleet of tugs, salvage ships, clean-up vessels and helicopters have patrolled the area where the tanker sank, 200 kilometres (120 miles) off the coast.

Some 500 workers are employed scraping oil from the coastline.

The Spanish director of the Greenpeace environmental pressure group, Juan Lopez, accused the government of not doing enough.

"It is not dedicating the means necessary for a catastrophe of this magnitude," he said.

Mr Rajoy said Spain was looking at ways of recovering some of the costs of the disaster by suing the tanker's captain, owner, and managing company.