Toxic algae has been found in the capital's first bathing spot in west London.
About 40 notices will be placed around Windermere with dozens more in other Cumbrian locations.
The council says there is blue-green algae on the lake which can be damaging for wildlife.
There are currently 245 algae blooms across water systems in Northern Ireland.
The algae - which can be highly toxic to animals - was found in Slipper Hill Reservoir near Foulridge.
Alderford Lake will be closed for safety reasons until 13 July at the earliest.
The town council has been advised by the Environment Agency to shut the lake due to health risks.
The algae can be harmful to both people and animals, Hull City Council officials say.
The alert follows reports a young pet died shortly after a visit to Coniston Water.
