Potentially toxic blue-green algae has been discovered in Windermere.
The Environment Agency has taken samples that confirm evidence of the micro-organism in the lake.
The algae can cause skin rashes or other issues if the water if accidentally swallowed. It can also be dangerous for dogs and other animals.
Signs have been put up at the affected bathing areas of the lake.
A South Lakeland District Council spokesperson said: “Blue-green algae can potentially make people feel quite ill if swallowed or even by just swimming in it.
“We want the public to be aware of the effects and are asking people to be extra careful and vigilant when using the lakes and rivers for recreational purposes during the summer months.
“Farmers and pet owners should also prevent livestock and animals coming into contact or drinking the affected water, as it can sometimes prove fatal for animals.
“People should observe any notices near affected areas and avoid contact or drinking water from areas identified.”
A temporary situation
Many standing waters are vulnerable to problems with blue-green algae, typically between June and November.
All blue-green algae blooms should be assumed to be toxic and the blooms vary in colour from discoloured green, blue-green, greenish brown, or reddish brown. The blooms do die eventually and the water returns to normal.