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."
Two people die after being recovered from the sea at South Fistral
UK Surf & Skate Expo to return with bigger line-up in Newquay
Methodist minister reflects on eight years in Newquay as she prepares to move on
Concerns raised over anti-social behaviour near Newquay's new play park
Police appeal after 64 sheep stolen near Halloon Roundabout
Council backs replacing South West Water with a publicly owned company
Two men killed in collision near the A30
Poundstretcher confirms opening date for new Newquay store
Skybus customers contacted over Newquay flight changes
Newquay RNLI crew rescue man and dog from harbour in early morning call-out
Van occupants ordered to leave Newquay car park by Friday afternoon
English Para Surfing Open attracts competitors from across the UK and Europe to Fistral
One person taken to hospital after crash closes Newquay road
Boardmasters outlines welfare and safeguarding measures ahead of festival
Newquay mayor backs launch of Beryl e-scooters in town
Cornwall's worst mining disaster to be commemorated
Newquay BID launches campaign to encourage visitors during Boardmasters week