The white glue-like substance which has killed seabirds along the south coast is an oil additive called polyisobutene.

Plymouth University has told the BBC that it has identified the substance, used as an additive in lubricating oils to improve performance.

Chemical analyst Professor Steve Rowland, of the university's Centre for Chemical Sciences, told the BBC all the data it had gathered supported the conclusion.

He said: "It's very sticky and semi-solid, hence its acting on birds' feathers like a glue."

He said the additive was relatively common, but he could only find evidence of one previous spill, in 1994.

Hundreds of birds have been washed up over the last week. The substance reduces their waterproof coating and ability to stay afloat at sea.

Dorset Wildlife Trust said the oil was "potentially harmful" to humans, although no injuries have yet been reported.

More than 300 birds have been treated at the RSPCA West Hatch centre near Taunton, Somerset.