The algae occurs naturally, usually between June and November
Warning signs have gone up on the shores of Windermere in Cumbria after the warm weather resulted in a bloom of potentially toxic algae.
South Lakeland District Council (SDLC) distributed the notices after being notified by the Environment Agency.
The blue-green algae, named after its vivid colours, occurs naturally and can be toxic to dogs and children.
While it is present in the lake, visitors are advised not to swim, or drink the water.
Tracy Howard, SLDC's environmental health team leader, said: "Blue-green algae can potentially make people feel quite ill if swallowed or even by just swimming in it.
"We just want people to be aware of the affects and are asking them to be extra careful when using the lakes and rivers for recreational purposes during the summer months.
"The public should observe any notices near affected areas and avoid contact or drinking the water."