A 100-metre-wide algal bloom left an area of Windermere unfit for bathing, tests have revealed, as campaigners blamed United Utilities for continually dumping sewage.

Matt Staniek, director of the Save Windermere campaign, said the group were alerted to an “extensive” area of blue-green algae at Waterhead, Ambleside on 25 June. The bacteria can be poisonous to both humans and animals.

Although it is not among the Government’s designated bathing sites, Waterhead is popular for water activities and people were swimming at the time, campaigners said.