One of the UK’s biggest water companies is fighting a legal battle to block public access to data on treated sewage it is discharging into Windermere in the Lake District.

United Utilities initially claimed that data from phosphorus monitors at sewage treatment works at the lake “was not environmental information”. It later claimed the information on phosphorus – which can pollute watercourses when at high levels – was “internal communication” and exempt from disclosure.

It is also trying to block the release of data on ammonia checks from a plant at Cunsey Beck where hundreds of fish were killed in a pollution incident in 2022 that was caused by an unknown source. The water company is fighting the rulings by the freedom of information watchdog to disclose all the data to the public.

Matt Staniek, the founder of the Save Windermere campaign, said it was “disgraceful” that United Utilities was repeatedly blocking the public from access to environmental information on potential pollutants. The water company has not commented on the costs to date of fighting the cases.

“United Utilities is withholding information which would tell us even more about the true impact of discharges of raw and treated sewage into Windermere,” he said. “There is an overwhelming public interest in understanding how a water company is polluting England’s largest lake.”

David Black, the chief executive of Ofwat, the water regulator, said earlier this year that he expected water firms to have a culture of transparency. “Customers have paid companies to install monitors and collect their data,” he said. “They have a right to see what it says.”

The legal battle comes as a new report obtained by the Observer reveals that United Utilities has presided over suspected illegal sewage spills for more than 500 days over a period from 2018 to 2023 in Windermere and the surrounding area. The analysis by expert Peter Hammond is based on data obtained by Save Windermere and the Windrush Against Sewage Pollution campaign.

Windermere is on the frontline in the sewage scandal that has polluted so many of the UK’s rivers, coastal waters and lakes. Residents in Windermere say England’s largest lake is now regularly blighted by blooms of potentially toxic blue-green algae, which are fuelled by sewage pollution.

Save Windermere requested data in January from phosphorus monitors on treated effluent at Windermere’s wastewater treatment works on the eastern side of the lake. It is concerned that treated effluent may be threatening the ecology of the lake, as well as raw sewage discharges. United Utilities refused to release the information under the environmental information regulations, classifying it as internal communications. It also tried to claim it was not environmental information.

It said the information was “non-regulatory” and was collected for operational purposes, and may not be as accurate as regulated sampling. It said it was not in the public interest to disclose the information because it might inhibit “free and frank” discussions and have a “chilling effect” on site management.

The Information Commissioner’s Office (ICO) ruled that the information should be disclosed, concluding: “If the phosphorus monitoring is accurate enough to be used for operational purposes … and to indicate the level of toxins present in the water, there is a public interest in this information.” The ICO has also ruled that United Utilities should disclose information it was withholding on ammonia sampling data and turbidity data (measuring cloudiness in water) at Near Sawrey wastewater treatment works on Cunsey Beck, a site of special scientific interest which flows into Windermere.

In June 2022, hundreds of fish were killed in a pollution incident at Cunsey Beck, which receives discharges from the sewage works. A cause for the fish kill was never established by the Environment Agency, but its investigation was later found in an independent report to have been undermined by a series of failings.

United Utilities is appealing to the first tier tribunal against the decisions to disclose the information requested by Save Windermere. A hearing on the cases, which are likely to be joined, is expected early next year.

Last month, United Utilities was reprimanded by the ICO for failing to properly handle requests from the public on environmental information. It said there had been numerous cases where “United Utilities has erroneously refused to deal with a request for information on the basis that the information being requested isn’t environmental”.

United Utilities said it is more committed to being more transparent and making information even more accessible. It says it will continue to engage with ICO processes.