The 9,000-tonne Dobrudja was escorted into harbour by a France navy vessel on Wednesday after a patrol aircraft saw a five-kilometre (three-mile) slick in the water about 230 kilometres off the French coast.

Tests were being carried out to establish if the pollution is from the ship's bilges.

The Dobrudja's captain, Yordan Golemanov, has denied the charge, according to the transport ministry in the Bulgarian capital Sofia which received a faxed message from the ship Thursday.

"After analysis of the photographs shown by the French authorities, it is clear there is no proof the pollution was caused by the Bulgarian ship. The only operation being carried out when the aircraft overflew was the washing of the deck with fresh water," Golemanov said in the fax.

The ship, which was en route to the Netherlands from the northern Spanish port of Aviles with a crew of around 20 Bulgarians, can only leave port on posting of 300,000 euros (340,000 dollars) in bail.

De-sludging -- the practice of dumping heavy oil residue from a ship's engines -- is estimated to cause far more environmental damage than accidental spills, and is punishable in most countries by heavy fines.

Golemanov could be made to pay 600,000 euros if he is found guilty, the French prosecutor's office said.

TERRA.WIRE