Schools have been closed and hospital appointments cancelled due to a burst pipe that has left 100,000 properties in London with little or no water.
Thames Water said the fault at its works in Hampton had caused problems in the west and south-west areas.
It said it was working “flat out to get water flowing” and was in the process of setting up bottled water collection points.
The TW, KT and W postcodes have been affected.
Twenty-seven schools in the boroughs of Richmond and Hounslow have been closed, including Trafalgar Junior School in Twickenham, which has been left without flushing toilets and washing facilities in the kitchen.
Surrey County Council also confirmed six schools had closed in Sunbury-on-Thames because of the issue.
Hounslow and Richmond Community Healthcare NHS Trust said all planned clinics and sessions at Teddington Memorial Hospital and Teddington Health and Social Care Centre were cancelled.
The evening fixture at Kempton Park Racecourse in Surrey has also been abandoned and, according to the Local Democracy Reporting Service, Chiswick Library and King Street in Hounslow are shut.
Ealing, Hounslow, Shepherds Bush and Hammersmith and Fulham have all been affected.
Thames Water said: “We’re working flat out to get water flowing out of our treatment works again, where a large pipe burst this morning.
“We’re pumping water into our pipe network from other areas to get supplies back to normal for as many of our customers as we can and are looking at providing bottled water for those who will be affected for longer.
“We fully appreciate how disruptive having no water is, especially for schools, and we’re really sorry that we’ve not been able to provide the service our customers expect of us today.”
It added it would put the locations of bottled water collection points on its website and social media “once they’re up and running”.
Mayor of London Sadiq Khan said it was “unacceptable” that so many people had been left without water “for several hours with little or no information on when supplies will be restored”.
He said he expected Thames Water to compensate all those affected.
Chelsea Willis, from Teddington in south-west London, said the lack of water had prevented her from feeding and bathing her six-month-old daughter Rhivér.
“My daughter has eczema so I have to bath her once a day,” the 29-year-old said.
“I called my housing association, who said they couldn’t help for 12 hours, but somebody there said they couldn’t let me go without and personally ordered three bottles of water and got it delivered to me.”
Tanya O’Connell, who lives in Twickenham, said the problem delayed her taking medication while she recovered at home following an operation last week.
The 37-year-old bank manager said her surgeon told her to take soluble pain relief, which she was meant to take at 08:00.
She said Thames Water “promised they would send someone with emergency stuff” but she had to eventually send her mother to the shop to buy water.
“It was difficult for her, she’s in her 60s with a bad leg… taking litres of water up the stairs,” Ms O’Connell said.
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