Blue-green algae which can kill dogs and make humans ill has been spotted in Ullswater near Pooley Bridge.
The warning comes as more tourists are expected to be visiting the Lakes as the lockdown eases and people make the most of the warm weather. There have also been sightings at Windermere at Brockhole, Derwentwater, Grasmere and Rydal.
The algae, which is also a danger to livestock and can make people vomit or suffer painful skin rashes, is a risk in lakes during sunny weather.
The Environment Agency has received reports of the algae, but has been unable to take samples and confirm the sightings as the testing lab is not operating in the lockdown.
Emma Brass, principal environmental health officer, said: “We are urging people to be cautious following these reports to the Environment Agency.
“Dogs should be kept well away from the water and if people come into contact with water that could contain the algae they should wash in clean water.”
Further information about algal blooms can be found on the internet