Islanders are being advised against swimming at Grève de Lecq after water samples showed high levels of E.coli.
Signs are being put up on the beach advising against bathing.
Anyone who has been in the sea there and starts to feel unwell should speak to their doctor.
The bacteria, which can cause diarrhoea, stomach cramps and fever, was discovered in a sample taken near Grève de Lecq this week. It returned a count of 3300 cfu/100ml, which officials say is 'considerably higher than the European threshold for “poor” water quality'.
The testing was part of the government's routine monitoring of bathing water qualityat 16 bays between May and September.
The Pollution Control team is investigating the cause if the high E.coli levels.
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