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Scientists investigating mass pollution which injured scores of sea birds in Dorset say the source of the contamination may never be known - some were also washed up in Hampshire.
Around 300 birds, mostly guillemots, were treated at the RSPCA West Hatch centre near Taunton, Somerset, over the last three weeks following the substance spill. Most are expected to be released back into the wild in the coming weeks following the pollution, which affected a 200-mile (322km) stretch of coastline.
But conservationists predicted the number of dead birds was likely to have been hundreds more, as many could have been blown out to sea by prevailing winds.
Experts at Plymouth University had previously confirmed the mystery substance was almost certain to be polyisobutene, an oil additive known as PIB which has a chemical mixture ranging from oils to solids.
But the Maritime and Coastguard Agency (MCA) said today that it had been unable to trace the source of the spill and confirmed it has closed the investigation.
In a statement, the MCA said:
''After tests were carried out on a sample of the product, it was identified as polyisobutene, or polyisobutyliene. This is a fairly common chemical carried aboard ships and it is produced in a large number of countries.
''Despite further tests, we have been unable to identify specific components of the product that may have helped us find the source.
''We did not receive any reports of pollution within the English Channel area at the time when the birds were coming ashore, but an MCA counter pollution surveillance aircraft surveyed the English Channel from Dover to the Isles of Scilly. In addition, images from the European Maritime Safety Agency's satellites were reviewed. No pollution was detected.
''As such, we have concluded that it is highly unlikely we will be able to link the pollution to any specific vessel. Unless we receive any new information, our investigation is now closed.''
Professor Steve Rowland, who led the Devon university's study, said the substance was not believed to be toxic.
But he added the substance could still prove fatal to birds. He said: ''When it is caught up in the birds' feathers, it just glues them together. That was one of the things that made it so difficult to establish what it was.''
Staff at West Hatch first tried to clean the birds with normal soapy water, which was not successful in removing the sticky substance.
Wildlife assistants then had more success removing the sticky gloop after they cleaned them with margarine.
At its height, the centre dealt with around 300 birds. Just over 60 were later transferred to another centre, while 90 died despite the best efforts of staff.
West Hatch spokesman Peter Venn said:
''We are looking to release the first batch of birds back into the wild in the next 10 to 14 days.
''It has been a gradual process to recovery.
''Unfortunately not all the birds have made it. But for those who have, we are looking for release locations in Dorset and the south coast of Devon.
''Although we don't know exactly where each bird was found, it is likely they would have drifted through the English Channel anyway. The important thing, once they are healthy enough to return to the wild, is that they are released so they can get back to the Channel.''