Forensic scientists had established the mysterious waxy substance covering more than 100 seabirds washed up on southern English beaches was a mineral-based oil, the Environment Agency said late on Friday.

That meant it could be anything from hydraulic fluid to Vaseline, scientists said, but was definitely not the palm oil originally suspected to have affected birds found from Cornwall to Sussex.

“The problem is, until we find the slick we don’t know why so many birds are being affected over such a wide area,” said Simon Boxall, of the school of ocean and earth science at the University of Southampton.

“We could be wrong but one would guess this might be more than just a simple operational leak,” he said. “It’s more likely to be something like a cargo vessel carrying the stuff that’s accidentally lost some over the side, or someone is washing tanks out at sea.”

Cargo ships are supposed to flush out their tanks when docked so they can safely dispose of any potentially hazardous substances, said Mr Boxall, but it is cheaper and faster to flush tanks illegally at sea.

The Maritime and Coastguard Agency dispatched surveillance aircraft to sweep the southern coastline on Friday as speculation mounted over the cause of the incident. “It’s quite a rare occurrence,” the agency said.

A sample of the substance found on the birds was sent to the Environment Agency’s forensic laboratory in Nottingham on Friday and the agency said staff would keep working on it until they could establish exactly what it was.

RSPB staff and volunteers were working along the coast on Friday to help distressed birds, most of which were guillemots.

Though most were alive, some had died, the RSPB said, adding the RSPCA had treated more than 100 birds, using margarine to remove the sticky substance in some cases.

“Some birds have reportedly been rescued with pebbles stuck to them or their wings stuck down,” the RSPB said, adding many of the birds had been found in clusters, including 100 found at Chesil Beach in Dorset.

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