Up to 5,000 homes in Bedfordshire have been left without water since Friday evening due to a faulty valve.
About 12,000 properties in Leighton Buzzard, Toddington, Hockliffe and surrounding areas were affected.
The valve was fixed on Saturday evening but properties in Leighton Linslade are still without water because of air in the system, Anglian Water said.
The firm apologised and said it was working to restore supplies.
It warned that water was unlikely to return to the areas until Sunday afternoon.
One resident said shops in the area had run out of bottled water.
Anglian Water said customers who were in its “priority list”, such as elderly people or families with young children, had been delivered bottled water.
Regan Harris, from the company, said: “Most of our customers will be coming back to water soon.
“There is an area on the northern part of Leighton Buzzard where people may be without water for a little while longer due to an air pocket.”