The stricken vessel was carrying 6,000 tonnes of chemicals, including 4,000 tonnes of styrene, a highly toxic substance used for making synthetic plastics.

The incident comes 10 months after the Maltese-registered tanker Erika sank off the coast of France, spilling thousands of tonnes of fuel oil and causing major environmental damage.

Environmental concerns

French President Jacques Chirac expressed "deep concern", and said "every means available must be used to prevent a new ecological disaster."

Oil from the Erika devastated the French coast earlier this year

It has also emerged that the Ievoli Sun was recently inspected and passed by the same Italian regulators who gave the Erika a clean bill of health.

Maritime sources say the styrene, which is highly toxic and corrosive, does not dissolve in water and should float to the surface, where it could be recovered by pollution-fighting ships.

But environmentalists say that in the worst-case scenario, leaking styrene would mean certain death for all affected flora and fauna.

Exclusion zone

A small oil slick has been reported one mile east of the vessel's position and small white spots have also been noted.

If this leaks into the sea then we could have a major disaster on our hands... this chemical is very hard to trace and anything that comes into contact with it will not survive.

Bruno Rebelle, Greenpeace

French officials have not yet established whether the ship's cargo is leaking, but a two-mile exclusion area has been established around the vessel.

Authorities fear poor weather conditions may prevent divers from examining the ship for another 10 days. It is believed to be lying intact at a depth of about 60 metres (200 ft).

French Transport Minister Jean-Claude Gayssot is heading to Cherbourg for crisis talks with maritime officials.

Waiting game

Officials on the island of Alderney are monitoring the situation closely, but a spokeswoman said it was too early to say whether the ship's toxic load posed a risk.

The Ievoli Sun was caught in ferocious storms

"As far as we are aware, most of the chemicals are still intact in their containers, so we hope we will not be affected by it," she said. "We've just got to wait and hope."

The 11-year-old Ievoli Sun is owned by the Italian company Marnavi. It was heading for the oil-processing area of Berre near the southern French port of Marseille, from a refinery at Fawley in the UK.

The ship was caught in the severe storms that raged across north-west Europe this week and radioed for help on Monday, reporting a hole in its double hull.

The crew were airlifted to safety, but the 114-metre-long (376 ft) tanker sank while authorities were attempting to tow it into the French port of Cherbourg.