Residents living in as many as 240 homes in a Berkshire town have been told not to drink their tap water.
Thames Water told people in London Road and Riverdale Court, Newbury, not to drink tap water, after a water main burst near a sewage pipe.
Bottled water was delivered to the homes, and samples of the mains water were taken to be tested.
The water company emphasised there was little chance of contamination and that the ban was a precautionary measure.
The company delivered letters to residents warning them that they should avoid drinking the tap water after it discovered the problem late on Tuesday evening.
Hilary Bennett, of Thames Water, said: "The tap water shouldn't be used for drinking or cooking. It can be used for washing and cleaning."
"Once we do have these sample results back we will notify customers as soon as we are confident that the water is safe to drink."