People visiting Guernsey's Cobo Bay are being told to avoid direct contact with the water at an outfall on the beach after traces of E-Coli were found.
Agriculture, Countryside and Land Management Services had previously said islanders should exercise caution in the bay after a serious of poor water quality results.
Having tested water from the outfall between Route de Cobo and the slipway near the kiosk, traces of the bacteria were detected.
E-Coli can cause gastrointestinal problems such as stomach cramps, sickness and diarrhoea.
The States say there is no link between the outfall and the foul drainage system, leading them to believe the source of the problem is further upstream.
Parents are advised against allowing children to play in water near outfalls and anyone entering the water is encouraged to practice good hand hygiene.