Beaches have been closed on Hayling Island after an unidentified yellow foam washed up along several miles of beach.
The foam, which is currently being tested by scientists, washed up today along the entire stretch of Hayling's southern coast, as well as beaches at the Witterings in West Sussex.
Havant Borough Council and Chichester District Council made the decision to close the beaches this afternoon while the substance is examined.
West Wittering Estate and Cakeham Manor have also taken this action and closed their beaches.
Signs have been put up to say they are closed until further notice.
Officers from the Environment Agency have taken away samples for tests.
Initial tests have shown that 80 per cent of the substance is Castor Oil, which is not a public health risk.
However, it can make animals sick if digested.
Initial tests show the remaining 20 per cent of the substance is oil mixed with seawater.
A spokeswoman for Havant Borough Council said: 'The beaches have been closed until further notice until the unidentified substance has been identified.
'We are investigating what it is and where it came from.'
A spokeswoman for Chichester District Council said: 'We are closing the beach as a precaution.
'The unidentified foam-like substance was discovered earlier today. The Environment Agency has taken samples to test.'
Paul Reynolds, senior environment officer at the Environment Agency, said: 'Our officers are on site sampling the material to find out what it is.
'As yet, there has been no apparent evidence of impact on wildlife.
'Once we know what the substance is we will be able to advise on action to take and appropriate disposal options for the material.'
Havant Borough Council said a clean-up operation will begin tomorrow with a view to re-opening the beaches as soon as possible.