People have been told not to go swimming at a popular bathing hotspot in the River Deben after high levels of E.coli were detected.
Testing was carried out in Ufford after concerns about water quality were raised.
The parish council said they found levels of E.coli well over the legal limit at Hawkswade Bridge, known locally as Ufford Hole.
"Ufford Hole has been popular with swimmers and children for years", said David Findley, chair of Ufford Parish Council.
"If you can't go to the beach, you go there.
"We tested last weekend and found very high levels of E.coli in one section where children jump in and swim.
"In the lower area we found 3,800 colonies/100ml, four times over the legal limit."
E.coli bacteria infections can be serious, often causing severe diarrhoea, sometimes with blood in it, abdominal cramps and fever.
Local testing for the bacterium started 12 months ago as part of a project led by Woodbridge Town Council member Eamonn O'Nolan and the University of Suffolk.
A number of local parish councils have been encouraged to take samples from areas of the Deben, from Debenham to Woodbridge.
"We've put up notices warning people not to enter the water", said Mr Findley.
"Heavy rainfall should raise the water levels and hopefully diminish the problem.
"But at the moment it's pretty dangerous for people to swim in."
The Environment Agency has said the River Deben is not a designated bathing water, highlighting that "current regulations for rivers and open waters in England protect wildlife and are not designed for the protection of human health".
A spokesman for the Agency said: “We welcome the growing interest of people using rivers and open waters for recreation.
“We do monitor E.coli levels at designated bathing waters. However, the River Deben is not a designated bathing water.”
“E.coli is a naturally occurring bacteria and is found in the guts of all warm blooded animals and humans. It can enter rivers from a number of sources including birds, dogs, cattle, run-off containing animal faeces and storm overflows.
“Anyone can become unwell when swimming in open waters.
"Public Health England and the EA offer advice in their ‘swim healthy’ guidance.”
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