Householders in parts of South Devon have been asked to boil their water as a precautionary measure because untreated water may be in the system.
South West Water has issued a notice to about 16,000 homes, businesses and schools around South Dartmoor and the east of Plymouth.
It is believed water from a stream may have infiltrated a water treatment works overnight.
People are being asked to boil water for 48 hours until given the all clear.
AFFECTED AREAS
Inspectors are examining the water network after being alerted by a remote monitoring system to a problem overnight.
It is thought there could potentially be untreated water in the supply system served by the Houndall Water Treatment Works.
Water in the affected areas can be used for washing, bathing and flushing toilets but residents are being asked to boil water for drinking, cleaning teeth, washing food, cooking and medical purposes.
A spokesman for South West Water said: "We apologise for the inconvenience this will cause and we are working to rectify the situation as soon as possible."
Plymstock school, which is in the affected area, decided to close on Friday and pupils were sent home. Some other schools have bought bottled water.
A spokeswoman for Plymouth City Council education department said: "It has been up to individual schools to make a decision on whether the arrangements are workable and manageable for them."
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