Increasing numbers of stricken birds are washing up on the south coast after being covered in a waxy substance.
Wildlife experts were last night no closer to discovering the cause of the environmental damage, which has seen more than 100 seabirds taken into care at the RSPCA West Hatch wildlife centre in Taunton, Somerset.
Most of the birds, guillemots, were found at Chesil Beach, near Portland in Dorset. One bird was found alive as far as Worthing in west Sussex, and is now being cared for at a veterinary surgery. Another was found in the Isle of Wight.
Scientists are working to find out what the substance is, but earlier suggestions that it could have been palm oil have now been ruled out.
Around 200 miles of the English coastline is being investigated. The Environment Agency has taken samples of the water for testing.
RSPCA deputy chief inspector John Pollock, who has been leading the rescue mission in Dorset, said: "We just do not know what this substance is. It is white, odourless and globular, like a silicone sealer. The best way I can think to describe it is like 'sticky Vaseline'.
"The numbers of the birds coming in have been growing and sadly there were quite a few dead birds this morning.
"We are still down at the beach, though, collecting and trying to save as many of them as we can. We are expecting this rescue mission to continue through the weekend."
A Maritime and Coastguard Agency (MCA) spokesman said: "This occurrence of seabirds being washed up on south coast beaches contaminated with a product is rare. We are working with partner organisations and agencies to deal with this event.
"The Environment Agency has taken samples of the product and is currently analysing it to identify it.
An RSPCA spokesman said they were pleased with the public's willingness to help the stricken birds, but warned of the dangers involved.
The spokesman said: "We would urge people to be cautious going down to the coastline affected.
"The instinctive reaction is to go down and look. But we don't know what this substance is, so our message is for people – especially those taking dogs down to the coastline – to please be careful."
The RSPCA centre at West Hatch took in 139 birds yesterday, but said 16 of those died. West Hatch manager Peter Venn said yesterday: "The current situation seems to be that the numbers of the birds coming in to us may have peaked for the time being, but that may change."
Supervisor Paul Oaten said they are using a process known to clear sticky substances in order to do the best for the birds.
RSPCA animal collection officer Steve Powell, 61, warned: "They do say that for every live bird that is taken off the beach there are probably nine others that have died at sea."