(Bloomberg) -- Water testing by an environmental group has revealed “alarming” levels of E.coli bacteria from sewage in the River Thames where the Henley Royal Regatta takes place next week.

The tests found as much as 25,000 colony-forming units of E. coli per 100 milliliters of water, more than 27 times higher than the level the Environment Agency grades as poor bathing waters, according to data published on Friday by River Action, a campaign group monitoring pollution. The bacteria can cause severe stomach cramps and illness.

Henley is preparing for its busiest week of the year hosting the internationally-renowned rowing competition — a six-day event from July 2 that attracts hundreds of thousands of visitors. This year it coincides with the UK general election on July 4. 

The contamination of the country’s rivers and waterways has angered voters. Henley, a Conservative stronghold for nearly 140 years, looks set to swing to the Liberal Democrats for the first time, according to a poll, based on new constituency boundaries.

Based on the testing locations, the sewage around Henley is likely to be from nearby treatment facilities operated by Thames Water Ltd, River Action said. The water samples were tested using a World Health Organization verified E.Coli analyzer and assessed by environmental charity Earthwatch.

So far, much of the public concern around sewage in waterways has focused on storm overflows across the wider sewer network. But treatment plants were responsible for 40% of all spills across England last year, pouring untreated sewage into waterways on average 64 times, over a combined 695 hours or 29 days, data analyzed by Bloomberg shows.

British Rowing guidance advises competitors at the regatta to cover cuts, grazes, and blisters with waterproof dressings and take care not to swallow any river water that splashes close to the mouth.

“This is a health emergency,” said James Wallace, chief executive of River Action. Thames Water needs to “stop this deluge of raw sewage, which threatens river users with serious sickness and the river’s biodiversity.” 

Thames refuted the findings, saying there were only two days each in May and June when readings spiked following rainfall.

“Our Sewage Treatment Works in the area have not released untreated effluent since 14 May, demonstrating that multiple sources are likely to have contributed to these elevated readings, which could include farming, industry, road runoff and wildlife,” a spokesperson for the utility added.

The UK’s water companies are facing public anger over illegal sewage spills and chronic leaks caused by decades of under-investment in aging infrastructure. Firms have proposed raising bills by an average of 35% from 2025 to 2030 to fund investments. The regulator is due to rule on whether it will allow the hikes on July 11. 

--With assistance from Eamon Akil Farhat.

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