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.

Samples which were taken at the site – part of England’s largest natural lake, in Cumbria’s Lake District National Park – were tested by algologist Dr Nick Everal.

They showed potentially harmful levels of cyanobacteria – also known as blue green algae – in the water.

In one sample, 809,298 cells of cyanobacteria per millilitre were recorded – more than eight times the World Health Organisation’s recreational limit of 100,000 cells.

On this occasion, toxins were not detected but Dr Everal warned that research suggests blue-green algae often produces them.

“Sixty to 70 per cent of blue-green algal blooms produce toxins and other by-products which can cause allergic skin reactions and vomiting sickness when ingested by humans,” said Dr Everal.

“People would do well to remember that, weight for weight, the toxins produced by these organisms are as toxic as cobra venom if ingested.”

Blue-green algae can occur naturally during warm, sunny weather, but experts said that blooms should not be a regular feature of the water in Windermere because its catchment is rocky and nutrient-poor.

Stephen Trotter, chief excutive of Cumbria Wildlife Trust, wrote last year that Windemere “should be clean, clear and low in phosphorus” and that he was concerned by an increase in “nutrient pollution”.

“Algal blooms in Windermere should be incredibly infrequent,” Mr Staniek told i. “What’s happened is likely a result of not dealing with the number one contributor to phosphorus in the water – which is United Utilities.

“United Utilities is still the single largest input of the nutrients that feed algae growth in Windermere.”

Last year, the water company – which serves the area – paid the highest dividend in the UK, at £320m, and has accrued £8.9bn in debt in the 35 years since privatisation.

United Utilities says it is investing in water infrastructure and has reduced the amount of phosphorus in Windermere by 30 per cent.

The Labour Government has set out its first steps to fixing the country’s sewage crisis and handing more power to consumers, including forcing water companies to reimburse customers if toxic spills are not tackled.

Ministers are expected to follow up with tough measures that will include banning the payment of bonuses to water bosses as well as placing the worst performing firms into “special measures”.

However, Save Windermere called on the Environment Secretary Steve Reed to install signage warning swimmers about algal blooms at the lake and to force the Environment Agency to carry out more regular testing.

“Labour have shown action and should be given some credit,” Mr Staniek added. “They are demonstrating a willingness for change but now it’s time for them to get on top of this stuff.”

A United Utilities spokesperson said: “Over the last two decades, we have invested £75m upgrading wastewater treatment sites, pumping stations and sewers around Windermere, with £45m of that work completed in 2020.

“Total phosphorus in Windermere has reduced by 30 per cent as a direct result of that investment.

“We are investing a further £41m into the Windermere catchment between now and 2030 to reduce storm overflow operation at Elterwater, Hawkshead, Ambleside and Near Sawrey by 50 per cent on 2022 figures.

“This is expected to reduce total phosphorus by a further 4 per cent and 8 per cent in the two basins of Windermere.

“We continue to work with all organisations that impact water quality in the lake including farmers, septic tank owners, hotels, campsites, highways authorities and landowners to deliver the step change that we all want to see.”

An Environment Agency spokesperson said: “We are absolutely committed to improving the water quality in Lake Windermere, which is affected by pollution from different sources, including wastewater treatment works, storm overflows, septic tanks and road and agricultural run-off.

“We continue to work closely with the local community and a range of partners to reduce pollution from all sources, as well as reviewing wastewater treatment works permits within the Windermere catchment.

“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.”