The captain of the 9,000-tonne Dobrudja was ordered to post bail of 300,000 euros (340,000 dollars) while tests are carried out to establish if the pollution is from the ship's bilges.

On Wednesday a five-kilometre (three-mile) slick was seen by a patrol aircraft in the water about 230 kilometres off the French coast.

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.

TERRA.WIRE