People in parts of Surrey are being told to boil their water before drinking it over fears it could contain E. coli.
Hundreds of postcodes in the Oxted area have been affected.
SES Water said an issue was discovered during routine tests, and “our teams are working to investigate the problem and restore supplies to their usual high standards”.
The scale of the contamination has not yet been confirmed.
The water company is awaiting further test results.
In the meantime people in the area are being told to boil all water and let it cool before using it for drinking, preparing food or cleaning their teeth.
Pets should also be given boiled tap water.
Boiled water can be kept in the fridge, and should be covered and used within 24 hours.