A sign with an expletive was placed near a creek where E. coli was allegedly found due to sewage.
It appeared near Martlesham Creek in Suffolk – on the River Deben estuary.
Eamonn O’Nolan, who said he was not responsible for the sign, carried out tests on the water and said high levels of E. coli were found in five places.
Anglian Water said it was “impossible to comment on the validity” of water quality testing carried out by citizens as they were not “very representative”.
The sign, which also referenced local MP and Environment Secretary Therese Coffey with the tongue-in-cheek phrase “proudly sponsored by”, has since disappeared.
A campaign group’s bid for the river in Woodbridge to be granted designated bathing status was turned down earlier this month.
Mr O’Nolan, who represents the Woodbridge Climate Action Centre, said he decided to do the tests “out of curiosity”.
He said he was advised how to carry out the tests by a woman who worked in the environmental toxicology department at the University of Suffolk.
Mr O’Nolan said he tested 13 locations along the river and he found “illegally high levels of E. coli in five of the 13”.
“We found noticeable levels in all of the others,” he said.
“The sources are the Anglian Water sewage treatment plants.”
Mr O’Nolan said people had been “getting ill”, although they did not have “documented examples”, but instead “lots of stories”.
He said he wanted “signs up warning people that they shouldn’t be going in the water”.
When asked if he was responsible for the new sign, he said: “It wasn’t me, absolutely not – but I do like it.”
A spokesperson for Anglian Water said: “We’re getting a lot of inquiries about the presence of E. coli in rivers and the sea following citizen science testing.
“Largely these tests aren’t very representative, as they don’t have the right controls in place, without knowing their methodology, it’s hard to know.
“Yes, they represent a moment in time, but don’t take into account lots of factors.”
The spokesperson said they would be giving local river groups “the right sampling equipment and knowledge to collect local water quality date in a useful way”.
Ms Coffey, Secretary of State for Environment, Food and Rural Affairs, and Conservative MP for Suffolk Coastal, has been contacted by the BBC for comment.
A spokesperson for the Department for Environment, Food and Rural Affairs said: “We are clear that sewage being discharged into our rivers is unacceptable.
“That is why we have put the strictest targets ever on water companies, requiring them to deliver the largest infrastructure programme in their history.
“The secretary of state has demanded action plans on every storm overflow in England, prioritising those that are spilling into bathing waters and high priority nature sites.”
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