Those responsible for a spill that could have caused the deaths of hundreds of sea birds along England's south coast could be prosecuted, wildlife experts say.

Thousands of birds have been washed to shore along coastline that stretches from West Sussex to Cornwall after being covered in a sticky, oily substance. Conservationists and volunteers raced to the shoreline to save as many birds as possible, and hundreds - mostly guillemots - are now being treated at RSPCA centres.

Scientists from the Environment Agency identified the mystery substance as a refined mineral oil, but not from an animal or vegetable-based oil and they have ruled out palm oil.

Tony Whitehead, from the RSPB, said investigations were continuing to establish what it was and where it came from. "It's a refined mineral oil, which is a colourless and odourless substance, and it's related to petroleum jelly," he said. "We don't know where it came from and we need to do a lot more testing on this substance to try and track it back to its source.

"There are people speculating it could be from a ship, that's possible but we just don't know yet. We need to look at what happened and if appropriate take legal action and also, frankly, shame the people."

One expert said the oil could have been discharged into the sea accidentally or deliberately. "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," said Dr Simon Boxall, an oceanographer at the University of Southampton.

"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."

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. 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.

The centre was caring for almost 170 birds, and while numbers of dead birds are as yet unclear, one RSPCA officer said that for every live bird that is taken off the beach there could be up to nine others that have died at sea. 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.

Environment minister Richard Benyon added: "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."