Schools have been closed and bottled water handed out to residents after a burst water main.
South East Water engineers are working to fix a burst main which has left thousands without water this morning.
Areas affected include Ditchling, Lewes, Wivelsfield and Barcombe.
Plumpton Primary, Chailey School, Barcombe CofE Primary and St Margaret's CoE Primary in Ditchling are all closed because of the issue.
A South East Water repair crew is on site but the firm is warning the project is a complex repair expected to take around six hours due to the location of the problem.
Residents can get bottled water from Ditchling Village Hall, Chailey School and Barcombe Village Hall.
South East Water said they were working hard to repair a serious burst on a large water main in South Chailey.
Andrew Ball, incident manager at South East Water, said: “The burst occurred on one of our 15 inch diameter water mains late last night.
“Our specialist technicians have now managed to change the flow of water around our network of underground pipes and supplies should be returning to customers in Ditchling, Plumpton, Keymer, Hassocks and Chailey.
“Customers in the Barcombe area may still be without water, although some supplies have returned, and our staff and contractors are on site carrying out repairs to the damaged main in order to restore supplies. Due to the complexity of the burst main, we expect the repair to take around four hours to complete.”