Spain's Atlantic coast was devastated by the oil

The French mini-submarine Nautile is set to resume work on the hull of the Prestige oil tanker, which sank off north-western Spain six months ago polluting large areas of coastline.

Ifremer, a French research institute which operates the Nautile, said it had signed a contract with the Spanish authorities to check the effectiveness of a previous operation to secure the wreck's leaking hull.

The Nautile's support ship, the Nadir, will set off from the Mediterranean port of Toulon later in May.

The submarine sealed 20 cracks in the wreck last winter - dramatically reducing its rate of leakage - but the repairs were only expected to last a few months because of corrosion and strong currents.

Spain has not yet decided whether to pump out the 35,000 metric tons that remain in the tanker's hold or to seal it with heavy casing.

The decision to resume the Nautile's operation came as fresh incidences of pollution were reported on the coast of the French region of Brittany.

The public prosecutor of the city of Brest said on Monday that tar balls found on beaches in the south of the Finistere department were probably from the wreck.

However, experts say the pollution could have been caused by oil which had leaked from the wreck some time ago and was blown back to land by strong winds.

Compensation

The Spanish Government said that by the end of March the cost of the clean-up to Spanish authorities alone had reached 328m euros.

PRESTIGE FACTS Lying in 3,500 metres (12,000 feet) of water 240 km (130 nautical miles) off Spain's north-western coast Split in two parts 4 km (two miles) apart Original 77,000-ton cargo Estimated 35,000 tons remain on board Click here to read about the Nautile mini-submarine

Deputy Prime Minister Mariano Rajoy said the government was preparing to sue those responsible.

"We must clearly establish who the real culprits are, and that must be done by the courts," he said.

The International Oil Pollution Compensation Funds are expected to pay victims up to 171.5m euros, about half of which will go to Spain.

Most of the rest of the money will probably have to come from legal action - against the ship owners, the Greek company that managed the vessel, the Swiss-based Russian commodity traders who chartered it or whoever provided them with insurance for their trading activities.

The Prestige sank on 19 November with a 77,000-ton cargo of fuel oil.

Slicks from the wreck polluted Spain's Atlantic shores, as well as large areas of coastline in neighbouring France and Portugal.

The pollution devastated the fishing and seafood industries, threatening the tourist trade and taking a heavy toll on wildlife.

The Nautile's first operation, which ran from 16 December to 14 February, reduced leakage from 125 tons a day to 1-2 tons.