A company has warned residents in nearly 400 postcodes in Surrey that their water supply could be contaminated with E. coli.
SES water said the issue was discovered during routine tests and teams are now working to investigate the problem.
As of today, homes in Godstone, Westerham, Redhill, Lingfield, and Horley are thought to be affected.
Further investigations are now taking place to establish the scale of the contamination.
Customers in and around Oxted are being advised to boil their tap water following the initial tests by SES Water, Surrey Live reports.
SES said: "The issue was discovered during routine tests, and our teams are working to investigate the problem and restore supplies to their usual high standards.
"While we await further test results which will confirm the scale of any contamination, customers should boil their water – and then 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."
A spokesperson for SES Water said: "The water company has issued the precautionary advice after routine testing at Westwood Treatment Works found that tap water quality might be affected.
"Customers in the affected areas are advised to boil water before using it for drinking, preparing food and cleaning teeth. A full list of all postcode areas can be found on our website using this link.
"Any pets in the affected postcodes should also be given boiled tap water.
"Boiled water can be kept in the fridge, and should be covered and used within 24 hours."
Escherichia coli (E. coli) bacteria normally live in the intestines of healthy people and animals and most types are harmless or cause mild diarrhoea.
However some strains can cause severe stomach cramps, bloody diarrhoea and vomiting, the Mayo Clinic explains.
You may be exposed to E. coli from contaminated water or food, especially raw vegetables and undercooked ground beef.
Healthy adults usually recover from infection with E. coli within a week.
Young children and older adults have a greater risk of developing a life-threatening form of kidney failure.
A full list of affected postcodes can be found on the SES Water website.
