Thousands of people across Greater Manchester have been told to boil their water until further notice after a "technical fault" at a treatment works.
There are 17,000 properties affected in parts of Tameside, Saddleworth and Greenfield, United Utilities said.
While the problem has been rectified, some water, not treated to the usual high standards, has entered the local pipe network.
United Utilities said: "For customers affected in those areas, your tap water should be boiled for drinking, food preparation and brushing of teeth.
"You can continue to use tap water for general domestic purposes such as bathing and flushing toilets. If your water looks dirty or cloudy, we would recommend to wait until the water has cleared before washing clothes.
"We have issued this advice as a result of an overnight technical fault on our equipment at Buckton Castle water treatment works, which supplies the area."
The company said customers will receive compensation automatically which will be paid in the next five working days.