A CITIZEN science project has uncovered “really concerning” pollution in England’s largest and most famous lake.

Four sites around Windermere in the Lake District were given “excellent” ratings in the Environment Agency’s last annual classification during the summer.

But findings from the Big Windermere Survey showed levels of harmful bacteria and phosphorus that fail water quality standards using data from 71 sample sites around the lake’s shoreline.

Tests to assess the prevalence of two types of bacteria that can cause illness were taken throughout the year, with more than 1,000 samples collected by 350 volunteers over two-and-a-half years.

The data shows that overall Windermere’s levels of bacteria give it a “good” water quality rating, but pollution gets worse in the summer, with the north-east, north-west and south-west areas of the lake having bacteria levels that would lead to a “poor” or failing rating for bathing water quality.

Much of the lake’s shoreline also failed bathing water quality standards for illness-causing bacteria in the summer and tests for phosphorus — a pollutant that causes algal blooms, damages aquatic wildlife and comes from wastewater and agricultural activities — exceeded expected standards.

Windermere is part of the Lake District National Park Unesco World Heritage Site but has been the focus of concerns over pollution.

Simon Johnson, executive director of the Freshwater Biological Association, which co-ordinated the citizen science project with the University of Lancaster, said: “The evidence is clear and must lead to decisive action to improve water quality.”

The Department for Environment, Food & Rural Affairs said: “This government shares the nation’s fury at unacceptable levels of pollution pumped into this national treasure.”