Holidaymakers and residents are being urged to avoid entering the stream at Mawgan Porth on Sunday due to a 'slurry pollution' incident.
The Environment Agency says it is working with a farm to contain the pollution, which has entered a stream that flows onto the beach.
In a statement posted online, the agency said six kilometres of the stream has been affected, but there have been no sightings of dead fish.
The agency's own Bathing Water Info page currently shows an ESO (Emergency or Storm Overflow) alert for the beach.
A spokesperson said:
"Our officers are working with a farmer in Cornwall to contain slurry pollution which entered a stream flowing onto Mawgan Porth beach. Six kilometres of the stream are affected, but no sightings of dead fish. If you're on the beach tomorrow, you're advised not to go into the stream."
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