Almost 200 surviving birds – mostly guillemots – are being treated at RSPCA centres along the southern coast after they were washed ashore covered in the white, sticky substance.
There was earlier speculation the coating might have been palm oil, but scientists working to identify it have since reported that it is a refined mineral oil and further analysis is under way.
A spokesman for the Environment Agency said: “The results show that it is a refined mineral-based oil mixture, but not from an animal or vegetable-based oil, which rules out palm oil.”
Most of the birds – which have been coming into RSPCA centres since Tuesday – were found at Chesil Beach, near Portland in Dorset, but have also been found in west Sussex, Cornwall and the Isle of Wight.
RSPCA deputy chief inspector John Pollock, who has been leading the rescue mission in Dorset, described it as like a ‘silicone sealer’.
Staff at the RSPCA West Hatch centre near Taunton, Somerset, have been treating the birds using margarine and washing up liquid to clean the substance from their feathers.
The charity’s animal collection officers said they have seen hundreds of dead birds washed up in coves and beaches along the 200 miles of the southern coastline.
Peter Venn, manager at West Hatch, said: “One of our inspectors said that he would have estimated that in one of the coves they were searching at least 100 birds were dead in the water. Obviously there are a lot of birds affected, what we don’t know is what is still out there.”
Stan Woznicki, the Maritime and Coastguard Agency’s head of counter pollution, said yesterday: “We have not received any specific reports of pollution within the English Channel area, but today we sent one of our counter pollution surveillance aircraft to investigate. It covered the sea areas between Dover and the Isles of Scilly, but no pollution was detected.
“Initial analysis indicates that the contaminant is a refined mineral oil and further analysis results are awaited.”