Hundreds of seabirds have died after being covered in a "refined mineral-based oil mixture", the Environment Agency has said.
Almost 200 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 mystery substance may have been palm oil, but scientists working to identify it 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". He 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 have 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 dead birds 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.
Environment minister Richard Benyon said: "I have spoken with the Maritime and Coastguard Agency (MCA) and every effort is being made to identify the cause of this problem. I'd like to thank everyone involved in helping the seabirds affected and it's thanks to their efforts that many have been cleaned up and now have a chance of survival."
Wildlife staff are expecting to see more guillemots and razorbills come ashore over the weekend and say it is crucial that the source is found.
The MCA confirmed the results from the sampling by the Environment Agency showed the pollutant is a refined mineral-based oil mixture. Stan Woznicki, the MCA's head of counter-pollution, said: "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."