Visitors to a nature reserve in North Herefordshire are being warned to avoid the water after blue-green algae was found.
Signage has been erected at Bodenham Lake to warn visitors.
The algae produce toxins which can cause skin rashes, nausea, vomiting, stomach pains, fever and headache if swallowed. Occasionally it can cause more serious illness such as adverse effects on the liver and nervous system.
The blue-green algae (cyanobacteria) occur naturally in inland waters, estuaries and seas and cannot be removed or treated. They thrive in warm conditions and are likely to come and go throughout the summer season.
Anyone who has come into contact with water containing blue-green algae should shower with fresh water immediately.
Anyone who has come into contact with affected water and has become ill should obtain medical attention.