People in parts of Claverham, Cleeve, Clevedon, Congresbury and Yatton are being told not to drink the tap water after parasites were discovered.
Bristol Water has told customers to use bottled water for the next 48 hours while the problem is tackled.
A spokesman said: “Customers in the Clevedon area are asked to boil their water before use until further notice.
“The boil notice is affecting properties with postcode BS21 and a very small number in BS49.
“The notice is likely to be in place for at least 48 hours but those customers affected will be informed directly when the boil notices are removed.
“One test result from the raw water quality samples taken at the Clevedon treatment works detected cryptosporidium.
“Therefore, the advice to boil water is a precaution.
“The treatment works has been taken out of operation and the water is being supplied from a different treatment works.
“We are supplying bottled water to vulnerable customers in the area and we are contacting schools directly.
“Bristol Water staff will be hand delivering notices to properties affected by the notice and those staff are happy to answer any questions you may have.”
Cryptosporidium can cause respiratory and gastrointestinal illnesses.
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