Water cut affects 7,000 in Rhondda and shuts 11 schools
About 7,000 customers in Rhondda are without water after a major pipe burst in Pontypridd.
It led to the closure on Friday of nine junior and infants schools, Porth Community Comprehensive and Tonypandy Community College.
Welsh Water said supplies would probably be back on between 22:00 GMT and midnight.
It blamed the large main burst on heavy rain causing the ground to move.
Supplies are hit in Llwynypia, Trealaw, Llwyncelyn, Penygraig, Tonypandy, Porth, and Clydach Vale.
Peter Perry, executive director of operations for Welsh Water, said it was a very complex repair, and more than 100 people were working into the night with the aim of restoring supplies on Friday.
Mr Perry said of the 7,000 customers affected, the company had managed to text just over 6,500.
Water company spokesperson Kevin Bateman said: "We can confirm that water supplies to 7,000 customers in parts of the Rhondda Valley have been impacted after a large strategic water main in Pontypridd burst.
"This was caused by ground movement during the recent very heavy rainfall."
Mr Bateman said customers on the company's "additional services register" were being supplied with bottled water.
"We would encourage families with young children to contact us on 0800 0520130 so that we can get alternative supplies of bottled water to them.
"We are doing everything we possibly can to restore supplies as quickly as possible," he added.
The company has apologised to customers for the inconvenience.
"We anticipate that supplies will begin to return to normal later tonight, and if this changes we will contact customers by text and automated messages to mobile phones and landlines," Mr Bateman added.
The Welsh Water helpline is 0800 052 0130.