Hundreds of seabirds have died after being covered in a “refined mineral-based oil mixture”.
Almost 200 birds – mostly guillemots – are being treated at RSPCA centres along the south coast of England after they were washed ashore covered in the white, sticky substance.
There was earlier speculation that the mystery substance may have been palm oil, but scientists have since reported that it is a refined mineral oil.
Environment Agency staff took samples from the affected water in an effort to establish the cause of the pollution.
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.”
RSPCA deputy chief inspector John Pollock, who has been leading the rescue mission in Dorset, earlier described the substance as “white, odourless and globular”.
Mr Pollock added: “It is like a silicone sealer. The best way I can think to describe it is ‘sticky Vaseline’.”
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.
Most of the birds – which have been coming into the RSPCA centre 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 animal collection officers said they had seen hundreds of dead birds washed up in coves and beaches along the 200 miles of the southern coastline.
West Hatch wildlife centre is caring for almost 170 birds, and while the exact number of has not yet been confirmed, one RSPCA officer said that for every live bird that is taken off the beach, there can be up to nine others that have died at sea.