Swimmers and paddleboarders have been advised not to go in the River Stour near Bures after a number of people became ill.
The warning comes as a number of people reported diarrhoea and vomiting symptoms after visiting the river at Bures late last month.
Among those were children and their families kayaking on the water, as well as paddleboarders.
A sign has been installed along the riverbank by the Bures Sportsground Committee warning people not to swim as a result of the sickness cases.
The Environment Agency has also conducted tests on different parts of the river.
The sign, titled "warning – danger to health", reads: "Bures Sportsground Committee have been advised that members of the public who have swum in the River Stour in Bures have come ill with vomiting and diarrhoea.
"Do not swim here until further notice."
A spokesman for the Environment Agency said test results have come back inconclusive.
The spokesman said: “We were recently made aware of people becoming ill from swimming at Bures.
"Our officers attended the site and carried out some sampling, including a test for E. coli. Four separate locations in the Stour, and on the stretch of river running through Bures, were tested.
“Unfortunately the results were inconclusive and did not identify the cause which made people ill.”
The spokesman added that people should visit the government website for guidance on swimming safely before making any decision on where to take a plunge.
Similar stories have surfaced in other parts of England in recent weeks, with norovirus-like cases stemming from swimming in rivers reported in Bristol, Manchester and in Staffordshire.
Other dangers of swimming in rivers and open waters have also been highlighted by the RNLI in recent weeks, with unseen currents and cold temperatures posing risks to people's safety.
More serious but rare consequences can also include leptospirosis, which is transmitted when a person's cut skin, eyes or mouth is in contact with animal urine in the water.
More information on how to "swim healthy" can be found here.
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