THE mysterious substance killing hundreds of seabirds off Dorset has been narrowed down to a 'refined mineral-based oil mixture'.
The Environment Agency released the findings tonight but is still not exactly sure where it came from.
Some 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.
Many have been washed ashore at Portland and the death toll in total so far along the coast has reached 200.
Rescuers will be out again this weekend but fear that the figure may rise.
Environment minister Richard Benyon has also praised the rescuers and the operation/ He said: ''I have spoken with the Maritime and Coastguard Agency 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.''
There was 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.”
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, 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 are tonight 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.
Peter Venn, manager at West Hatch, said: ''What we are hearing are reports of birds showing up on the Sussex coast, so that may mean that the weather is pushing them more easterly.
''This certainly adds up from the basis that we were getting birds earlier in the week from Cornwall, Devon, then up through Dorset and have had birds from Hampshire today, so everything is certainly not over by any means.
''There were quite a lot of birds dead in the water this morning. One of our inspectors said that he would have estimated in one of the coves they were searching at least 100 birds were dead in the water.
''Obviously there are a lot of birds affected, what we don't know is what is still out there.''
The Maritime and Coastguard Agency (MCA) said it is regularly called upon to react to a wide range of incidents and to develop a response to deal with emergencies at sea that cause pollution or threaten to cause pollution.
A spokesman added: ''This occurrence of seabirds being washed up on south coast beaches contaminated with a product is rare and we are working with partner organisations and agencies to deal with this event.''
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.
In the meantime they said they are doing all they can to help the birds already brought in to the centre at West Hatch, which earlier received a delivery of more than 500 bottles of washing up liquid.
Supervisor Paul Oaten has been washing the birds in the centre's dedicated cleaning room using vegetable oil and margarine, followed by washing detergent.
''They're coming up very well,'' he said.
''There's a couple of little patches that haven't proofed - where the water beads off the feathers - but we will do these birds in two washes, so we do a pre-clean to get the 99% off and then a final clean to get them 100% clean.''
RSPCA animal collection officer Steve Powell, 61, has been working with oiled birds for 25 years and took 23 birds in to West Hatch this morning.
''It initially started on Tuesday and has gradually increased on each day,'' he said.
''We saw quite a few birds waiting to be collected. They were just waiting for the tide to go out, but there were also a number of dead ones, unfortunately.
''The ones we've brought in today are a lot thinner than the ones we first collected on Tuesday. Whether they have been fighting at sea against the pollution, or how long they have been out there without feeding, I don't know. They are quite lively, but also quite thin.
''They do say that for every live bird that is taken off the beach there are probably nine others that have died at sea.''
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.''
The oil may have been discharged into the sea accidentally or deliberately, an expert said.
''Ships use lots of different mineral oils for working, you've got oil in the engines, a lot of hydraulic fluids, in the anchor winches, ship's cranes and so on,'' said Dr Simon Boxall, an oceanographer at the University of Southampton.
''Some kind of accident such as a leaky gearbox or a broken pipe is the most likely cause of a mineral oil spill, but it is unusual that it would cause this much havoc.
''So that makes one think it could be an illegal dump of cargo oil - oil that is being transported rather than used in the working of the ship.
''This could happen if there was illegal washing out of tanks at sea. Some unscrupulous operators, and they are rare, flush the tanks out there because it is cheaper and easier than doing it in dock. It is illegal and quite harmful.
''Until the Coastguard Agency can pinpoint the slick, and the size of the slick, it will be hard to know. Tracking it down is no easy task. A small slick is more likely to indicate an accident, a larger slick may mean oil has been dumped.''