Up to 10,000 seabirds could be caught in an oil slick from a ship wrecked off the Devon coast, the RSPB says.
About 1,000 affected birds have been collected since the MSC Napoli was beached off Sidmouth at the weekend.
Around 600 of these are likely to die, according to the Royal Society for the Protection of Birds.
But the charity said that it did not know what the final total of affected birds would be, and that it could be up to 10 times the known numbers.
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We just do not know how many birds have been affected by the oil
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A new slick has been formed from oil leaked from the stricken container ship.
The Maritime and Coastguard Agency said the slick, which started on Tuesday, was several kilometres long and 30 metres wide.
The agency does not think any more fuel is leaking, as work continues to pump 3,500 tonnes of oil from the vessel.
The RSPB spokeswoman Sophie Atherton said the true extent of the contamination of seabirds would be difficult to measure.
She said: "For every oiled bird ashore, there could be up to 10 times that number at sea.
"We just do not know how many birds have been affected by the oil."
The RSPCA said more than 500 seabirds had been treated at its West Hatch centre in Taunton, Somerset.
The birds were contaminated by between 60 and 200 tonnes of fuel oil which leaked from the Napoli.
RSPCA spokeswoman Jo Barr said the birds had been collected from Paignton in south Devon to Chesil Beach in Dorset.
This weekend a small army of RSPB staff and volunteers will scour about 100 miles (160km) of coastline and beaches between Slapton Sands in south Devon and Portland in Dorset for more oiled birds.
Water sample results taken this week by the Environment Agency near the ship so far show no signs of pollution from the oil, pesticides or other chemicals from the vessel.
Further sampling will be carried out twice a week for the foreseeable future.
The Napoli was grounded off Sidmouth last weekend during a tow to Portland because it was feared she could sink.
The ship was damaged during a storm in the English Channel.