Campaigners say a stretch of river used by the Henley Royal Regatta is being “severely impacted” by sewage waste.
Dozens of local volunteers supported by River Action tested the waters of the Henley Mile in the build-up to the annual rowing event in June.
They found levels of phosphate and nitrate - nutrients that can trigger damaging algal growth - were double those considered to be ecologically harmful for rivers.
There was also a spike in e.coli and other intestinal bacteria, following rainfall.
The spike in e.coli was sufficient for the river to be considered poor for bathing. Henley Mile is frequently used for wild swimming and rowing and last month five teenage boys reported vomiting, diarrhoea and stomach pain after swimming there.
Olympic legend Sir Steve Redgrave, chairman of the Henley Royal Regatta, said: “We are deeply concerned about the impact that sewage pollution is having on our beautiful river.
“I canoe from my home town of Marlow at Longridge. There’s a water treatment plant there and sometimes you can see from the colour of the water that there has been a discharge - it’s not somewhere I would ever want to fall in.
“We have to improve this situation so that we can preserve wildlife and make our rivers safe for everyone.”
He and TV naturalist Steve Backshall are backing River Action’s campaign to clean up the Thames.
James Wallace, chief executive of River Action, said: “The results of this prove the dire state of England’s capital river. Wildlife is collapsing and river users are at serious risk.”
Thames Water insisted that pollution could come from various sources, including agricultural land, and that ultra violet treatment to kill bacteria would not address this.
It said: “Taking action to improve the health of rivers is a key focus for us and we will review the findings of this report. We have an upgrade planned for Henley sewage treatment works which will provide a major increase in capacity and is expected to complete in 2025.
“We’re also supporting the local community’s application for a designated bathing water site in Henley.”
The Environment Agency said: “We are absolutely clear that polluting our seas and rivers is unacceptable. We are holding the water industry to account on a scale never seen before, with 58 prosecutions of companies, including Thames, since 2015 for environmental crime.”
