Up to 40,000 homes could lose water in two counties and two schools have been forced to close due to a burst main at a treatment plant.
The incident at the Bryn Cowlyd Water Treatment Works in Dolgarrog, Conwy, is affecting people in Conwy county as well as Denbigh and surrounding areas.
The firm said about 8,000 households lost supplies on Wednesday night and around a further 33,000 properties were now at risk of losing supplies until repairs were complete.
Ysgol Aberconwy and Ysgol Eglwysbach, both in Conwy, will remain shut on Thursday as they have no water. The problem is not expected to be fixed until later on Thursday.
Welsh Water said that tankers were being mobilised to keep as many people on supply as possible.
The company added that its crews were working on repairs, but the work was “difficult and dangerous” because the burst water main was two-and-a-half metres underneath the river bed.
Crews would have to create a dam-like structure in the river to be able to excavate and access the damaged water main, the company added.
The incident is affecting supplies in Conwy, Dolgarrog, Eglwys Bach, Groesffordd, Gwytherin, Henryd, Llanbedr y Cennin, Llanddoged, Llanfair Talhaiarn, Llangernyw, Maenan, parts of Llanrwst, Pandy Tudur, Pentrefelin, Rowen, Rhyd y foel, Tal y Bont, Tal y Cafn, Tyn Groes and Trofarth.
Welsh Water said customers had been contacted directly by text message to inform them about the issues.
