beach patrols have been launched to keep people out of the sea.
Council staff are monitoring the sands between Saltburn and South Gare, Redcar, east Cleveland, advising bathers to keep out of the water, following the discovery of an unidentified substance off the coast.
The Environment Agency is examining samples of what is thought could be an algae.
A spokeswoman said: "We are making every effort to discover exactly what it is. We do not know for sure that it is algae."
The agency was called in by the coastguard at the weekend and has been working with Cleveland Police and Redcar and Cleveland Borough Council.
Warning signs have been put up along the beach.
Paul Kirkham, the council's director of neighbourhood services, said: "We are still monitoring the situation. We would like to stress that this substance has not caused any harm to any person using the beach, or any wildlife in the area.
"Although this is not believed to be hazardous, it is possibly a naturally occurring algae that could cause irritation upon contact with the skin."
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