Water company United Utilities has been accused of failing to report more than 100 million litres of raw sewage that it illegally dumped into Windemere over a three-year period.

Analysis by the BBC found that the company, which serves seven million customers across the North West, pumped between 143 million to 286 million litres of waste into the lake between 2021 and 2023 at times when it was not permitted.

United Utilities also did not report at least 102 million litres of that pollution to regulators, the broadcaster claimed.

It comes just a day after the Information Commissioner’s Office (ICO) ordered the firm to improve its handling of requests for environmental information following complaints about a lack of transparency.

The importance of Windermere on a national and international scale is just enormous and this is just blatant exploitation

Although they are not subject to freedom of information (FOI) requests, water companies have a legal obligation to make information about the environment available under the Environmental Information Regulations (EIR), both proactively and if requested by the public.

The data analysed by the BBC was first requested from United Utilities under these regulations by anti-pollution campaigner Matt Staniek, it is understood.

While the company is authorised by the Environment Agency to release untreated sewage into Windermere under certain circumstances during wet weather, the broadcaster found that the Glebe Road pumping station, which discharges directly into the lake, had failed to comply with the permit.

The firm is meant to notify the Environment Agency but United Utilities reportedly failed to do this for some of the discharges identified by the analysis over the three years.

Mr Staniek, founder of Save Windermere, said the lake is being treated as “an open sewer” as he spoke about its cultural significance in the works of artists and writers such as Beatrix Potter, Arthur Ransome and William Wordsworth.

“The importance of Windermere on a national and international scale is just enormous and this is just blatant exploitation,” he said.

“It is illegal spilling, it should not be happening and this is why today we are calling on Keir Starmer to end pollution in Windermere once and for all.”

He added that United Utilities is “either very incompetent and they have been unable to demonstrate they can perform their statutory duty” or “it’s something more sinister and they knew about it”.

“They should have known about this,” he continued. “You have to ask the question why. Why haven’t they reported it and why hasn’t the Environment Agency done anything about it?”

Commons Leader Lucy Powell told MPs on Thursday that sewage in Windermere is an “appalling scandal”, calling the BBC revelations “shocking”.

She called for a Commons debate about the “ongoing appalling behaviour of water companies”, adding: “It’s home to countless species of wildlife and surrounding habitats, and it’s a haven for swimmers and water sports enthusiasts.

“The failures of United Utilities are clear for all to see, unlike the water they are polluting.”

United Utilities said they reported all discharges including any potential non-compliant discharges to the Environment Agency through “our reporting processes and liaison meetings”.

It also claims to be “industry leading in self-reporting” as it self-reports more than 94% of potential pollution incidents to the regulator.

A spokesperson said: “As part of our ongoing work to reduce spills, in January 2024 we discovered an engineering issue on the configuration of the Windermere system that meant some of the discharges recorded by our event duration monitors were potentially non-compliant.

“Even though these spills were happening when it was wet and our storage system was already full and spilling, we reported them to the Environment Agency as soon as we became aware of the situation.

“We have now implemented engineering changes to the system to prevent a reoccurrence.

“In addition, we are starting construction around Windermere next month to cope with increased rainfall, population growth and the impacts of climate change, which includes additional capacity and treatment over and above our permit requirements that will store more storm water and reduce discharges.”

The BBC previously revealed that United Utilities had downplayed dozens of polluting incidents, after which United Utilities retrospectively reported some discharges into the lake from October 2023 onwards.

But this latest analysis covering data back to January 2021 suggests illegal discharges had been taking place far longer than these retrospective reports.

We are absolutely committed to improving the water quality in Lake Windermere

The Environment Agency said in a statement it launched a “complex and ongoing” investigation into the sewage discharges from October 2023, adding that it is examining further evidence received from the company.

A spokesperson said: “If any water company is found to be in breach of an environmental permit, the Environment Agency will take the appropriate enforcement action up to and including a criminal prosecution.

“We are absolutely committed to improving the water quality in Lake Windermere, which is why we are working closely with the local community and a range of partners to reduce pollution from different sources.”

It added that teams have completed nine United Utilities asset inspections in the Windermere Catchment this year, with a further three inspections before the end of 2024.

Meanwhile, water industry regulator OfWat, which judges firms on the number of pollution incidents they cause each year and financially rewards or penalises them on their performance, said the allegations are “very serious”.

A spokesperson said: “Where evidence supports further action, we will not hesitate to use all the powers at our disposal.”

“In July, we opened an enforcement case against United Utilities as part of our ongoing enforcement activity against all 11 water and wastewater companies.

“As part of this investigation, we will consider whether they are fulfilling their obligations to protect the environment and minimise pollution.”

If a firm is found to be in breach of their obligations, Ofwat said customers would get their money back through underperformance penalties.