A tanker carrying 10,000 tonnes of phosphoric acid has sunk in the English Channel.
The vessel was badly damaged and listing after it collided with a cargo ship around 30 miles north-west of Guernsey early yesterday.
It is understood the tanker sunk shortly after midnight after it had developed a greater list.
A one-mile exclusion zone has been set up around the wreckage.
French rescue workers had been hoping to tow the vessel to Le Havre, in France, before it went down in one of the busiest shipping lanes in the world.
The operation was suspended late last night as it was deemed "too dangerous" to continue in the dark.
Twenty-two crew members on board the Ece tanker, which is registered in the Marshall Islands, in the Pacific Ocean, were rescued yesterday by British coastguards, the RAF and their French counterparts nearly two hours after they abandoned ship.
They were taken to hospital in Guernsey with mild hypothermia and shock, but otherwise uninjured.
Mark Clark, of the Maritime & Coastguard Agency, said: "The tanker sank after the listing got progressively worse.
"A one-mile exclusion zone has been placed around the area, with warning signs pointing out the wreckage to other vessels."
He added that UK officials would travel to the French rescue headquarters at Cherbourg today to assess whether anything can be salvaged from the wreck.
Surveillance aircraft will also attempt to gauge whether the acid has leaked into the sea, although it is not thought to pose a serious pollution threat to the environment.
French divers earlier examined the tanker and found its hull was intact.
One crew member said the side-on collision happened as both ships ran parallel to each other.
The Maltese cargo ship, General Grot-Rowecki, was only slightly damaged in the collision, which happened outside the traffic-separation scheme.
The scheme is a stretch of water in the middle of the Channel akin to a motorway, with ships travelling in two lanes, north east and south west.
About 500 vessels pass every day through the Channel, which is renowned as one of the busiest of its kind in the world.
Portland Coastguard were told the ship had sunk 30 miles north west of Guernsey at about 2.30am.
A spokesman for Portland Coastguard said: "It's not a danger to shipping. It's gone down in about 60 metres of water - that's pretty deep.
"It's not a hazard to shipping but the French have imposed an exclusion zone one mile around that position, so they have stopped fishing and all nautical activities."
He said the 10,000 tonnes of phosphoric acid would not release into the sea immediately but over a period of time.
"It's totally water soluble and will disperse," added the spokesman. "It's not like oil that stays separate, it will be mixed with water and diluted fairly rapidly."
Phosphoric acid is used in the manufacture of fertilisers, detergents and pharmaceuticals. It is also used to flavour fizzy drinks, beer, jam, jelly and cheeses. In food, it adds a tart, acid flavour.
It is used to produce water softeners in detergents. An increase of phosphate in water stimulates aquatic plant life growth.
David Santillo, a scientist at the Greenpeace research laboratory in Exeter, said phosphoric acid was a "hazardous substance" and attempts should be made to recover it from the sunken tanker.
"If there had been a major spill we could have been looking at more severe effects on the marine life locally, but with a slow dilution it shouldn't pose a long-term problem."
But, he added: "We shouldn't simply accept that it is going to leak into the sea water over time. Any acidifying of sea water can have quite profound effects on a number of marine organisms. The scale of the impact is difficult to predict.
"To say there would be no effect is to be a little economical with the truth."
Mr Santillo said phosphoric acid was a source of the nutrient phosphorous which could encourage unwelcome plant life growth that harms rather than enhances the local environment.
"Nutrient pollution is pollution in itself and it can have a lot of unforeseen consequences, generating the wrong sorts of organisms and harming marine life rather than enhancing it.
"Anything you introduce to the marine environment that shouldn't be there is generally going to have some unforeseen and adverse consequences."
He also said the tanker was probably carrying up to 1,000 tonnes of fuel oil which would have a "long-term" and "devastating effect" on marine life on the sea floor and on the water surface.
"We need to know how much is on board and whether there is any way of retrieving that," he added.Reuse content