The Ece is lying about 70m downA tanker which sank in the English Channel with its 10,000-tonne load of phosphoric acid is leaking fuel oil.
The Ece had about 80 tonnes of fuel on board when it sank on Wednesday while being towed to Le Havre.
The vessel had been badly damaged following a collision with a cargo ship off the north west coast of Guernsey early on Tuesday.
A plane crew spotted what they described as a "reasonable quantity" of fuel oil bubbling from the wreck.
Greenpeace said it feared the fuel oil could have a "long-term" and serious effect on marine life.
David Santillo, a scientist at the Greenpeace research laboratory in Exeter, said that with a slow dilution, the phosphoric acid should not pose a long-term problem.
But the fuel oil posed a serious danger to marine life on the sea floor.
He told BBC News: "It's potentially significant. The scale of the problem will depend on sea currents and how much time it takes to leak out."
Officials from France, Channel Islands and the UK are being consulted.
The Ece sank near the site of the collision, in 70m (230ft) of water, 90km (56 miles) west of Cap de La Hague.
French divers had found a 5m (16ft 4in) hole below its waterline, although there was no leak of the phosphoric acid.
French authorities have now imposed a one-mile exclusion zone with warning signs pointing out the wreckage to other vessels.
Crew rescued
The alarm had been raised at about 0220 GMT on Tuesday when the Ece collided with the freighter General Grot-Rowecki about 50km (30 miles) north-west of Guernsey.
Twenty-two crew members of the Ece were rescued unharmed.
The Maltese-registered carrier, carrying 26,000 tonnes of phosphorus, was only slightly damaged.
About 500 vessels every day pass through the Channel, one of the busiest shipping lanes in the world.
French fishing trawler Kleine Familie sank early this month, with the loss of five crew members, after colliding with a cargo ship in the same sector of the English Channel.
