A short stroll from Beatrix Potter’s former farmhouse in the Lake District are the waters of Cunsey Beck, nestling in the breathtaking landscape that inspired the tales of childhood favourites Jeremy Fisher and Jemima Puddle-Duck.

Campaigners say the once clear waters are regularly blighted by raw sewage from a nearby works. New figures obtained by the Observer reveal the Near Sawrey plant is alleged to have illegally discharged untreated sewage on 56 days from 2021 to 2023.

Matt Staniek, from the campaign group Save Windermere, said: “Beatrix Potter was in awe of this natural landscape. If she was alive today she would be campaigning to stop what has become a national disgrace.”

The discharges are revealed in a new analysis which shows that seven United Utilities sewage plants and pumping stations in the Lake District – including a station estimated in a BBC report last month to have pumped 140 million litres of waste into Windermere in three years – are alleged to have illegally spilled on 501 days from 2018 to 2023. The analysis by expert Peter Hammond is based on data obtained from United Utilities and the Environment Agency by Save Windermere and the Windrush Against Sewage Pollution campaign.

Hundreds of fish were reported killed in a pollution incident at Cunsey Beck in June 2022, but the Environment Agency failed to identify a definitive cause in what was later found to be a seriously flawed investigation.

The charity WildFish reported in September that sewage pollution at Cunsey Beck was causing the stream to deteriorate. It has written to the Environment Agency, urging it to order United Utilities to prevent further damage.

Keir Starmer said before the general election that he was sickened by the reports of raw sewage blighting Windermere. Campaigners now want the prime minister to back an ambitious infrastructure project to end the discharges into England’s largest natural lake. A similar project at Lake Annecy in France in the 1960s and 1970s saw it transformed into one of the cleanest lakes in Europe. United Utilities estimated in a report in February that a similar scheme at Windermere could cost up to £6.4bn.

Windermere is regularly blighted by blue-green algae, partly fuelled by discharges of untreated sewage and warmer weather. Agricultural and urban runoff also contribute to pollution. The algae can be toxic, and also suck up oxygen in the water, suffocating fish and other aquatic life.

Tony Coldwell, 67, a cinematographer from Holmfirth, West Yorkshire, who was sailing on Windermere last week, said he had also visited Annecy after it was transformed by the scheme to prevent sewage discharges. “Windermere should be as good as Annecy, which is crystal clear,” he said. “United Utilities has been taking the money for shareholders and not putting it into the lake. It’s a major scandal.”

Chris Taylor, 59, from Grasmere, who also sails on Windermere, was shocked at the algae blooms on the water in early November. “I’ve never seen it at this time of year like this,” he said. “We are basically paying the water company to pollute the lake.”

Hayley Leece, 42, a wild swimmer who lives in Ambleside, said she went swimming recently and found the lake was covered in algae bloom. “It was disgusting,” she said. “I could see the algae swirling around me.”

Environment Agency officials say that key pollutants in Windermere have declined since the 1990s, but more work needs to be done to tackle all sources of pollution. The agency works in the Love Windermere partnership, which aims “to bring about a healthier future for the lake”. More than half of the phosphorus in Windermere comes from sewage from United Utilities and private sewage works, according to the agency.

The latest analysis by Hammond, a former professor of computational biology, has found that four sewage treatment works and three pumping stations are suspected of breaching permits on 501 days from 2018 to 2023 by discharging raw sewage. Some of the data was withheld, with the firm citing potential prejudice to ongoing Ofwat and Environment Agency investigations.

Hammond’s work is criticised by United Utilities for being based on “assumptions”. Hammond said: “Ofwat recently cited my analysis extensively in notices for enforcement orders and financial penalties on Thames Water and Yorkshire Water. My analysis uses actual water company records and is published online for all to see. United Utilities has never published any analysis in rebuttal.”

United Utilities has refused to disclose data on phosphorus monitoring at Windermere waste water treatment works and other data from the Near Sawrey plant. It is appealing against rulings by the Information Commissioner’s Office to disclose the data.

Responding to the findings in the latest analysis, Staniek said: “This shows the exploitation of England’s largest lake, which has been used by United Utilities as an open sewer to dump untreated sewage.” Staniek said he was concerned that treated sewage was as much of a threat as untreated sewage to the ecology of the lake, and it was vital that United Utilities provided open access to environmental data.

United Utilities has announced nearly £200m of funding to improve and protect water quality at Windermere. It will improve treatment processes at wastewater plants and invest in improved storm overflows to reduce spills.

Campaigners believe a more radical solution is required, and want an end to all discharges of raw sewage and effluent. Stanley Root, who has studied successfulenvironmental improvement projects at Lake Annecy and Lake Washington, near Seattle, said a piecemeal approach was “doomed to failure”.

Root said a major infrastructure project to divert sewage flows would be worth the investment. “A crystal-clear Windermere would produce enormous long-term benefits,” he said.

The Environment Agency said: “We are currently conducting several investigations into pollution-related incidents in and around Windermere. We remain absolutely committed to improving the water quality in Windermere. Progress is being made, with recent improvements to sewage treatment works leading to a fall in phosphorus input by approximately 30% since 2020.”

A spokesperson for the Department for Environment, Food and Rural Affairs said: “This government will never look the other way while water companies pump record levels of sewage into our rivers, lakes and seas – and that includes the iconic Windermere.” The spokesperson said the government had introduced a water (special measures) bill to strengthen regulation and launched the largest review of the water sector since privatisation.

A United Utilities spokesperson said: “We understand people’s concerns about the operation of storm overflows and we are already significantly reducing their use, alongside proposing the largest environmental investment in a century across the north-west over the next five years.

“In Windermere, we have so far invested £75m in upgrading wastewater treatment sites and pumping stations and have proposed a further investment of £200m over the next five years to tackle and reduce spills from all storm overflows in the catchment.

“We continue to work with all organisations and individuals that impact water quality in the lake to help deliver the step change we all want to see.”

The firm said the Near Sawrey works, and another nearby plant, only served 370 people, and both operated in line with environmental permits.