Potentially toxic blue-green algae has been found in three locations in the Lake District.
The algae is always present in lakes, but the warm weather has made it bloom, giving off toxins which can be harmful to wild animals, livestock and pets.
It can also cause rashes and vomiting in humans coming into contact with it.
The Environment Agency said it has been reported in Ullswater, Coniston Water and Windermere. Warning signs have been put up.
An incident room has also been set up in the agency's headquarters in Penrith, and it is working with water companies, businesses and farmers to minimise the impact.
Jez Westgarth, from the agency, said: "We are getting reports of fish in distress and dying.
"For animals it's not great, and there's quite a lot of messages out there on social media about being careful with your pets.
"We've recently been increasing our monitoring and working with local authorities to ensure the message is spread."