Poisonous algae forms in Bodenham Lake Nature Reserve
People visiting a Herefordshire lake are being warned about a poisonous blue-green algae that has formed in the water.
The naturally occurring "blooms" at Bodenham Lake Nature Reserve release a toxin in to the water as they decompose, the council said.
They can be fatal to humans and animals if enough are ingested, a spokesman said.
People are urged to stay out of the water until the algae has subsided.
The blooms are caused by temperature changes in the water and usually appear after a warm, wet period of weather in late summer or autumn, the council said.
They usually disappear in cold weather.
Swallowing the water or algae can cause stomach upsets and skin irritation.