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 repairs to the pipe would continue throughout the night but that a bypass and water from elsewhere has meant supplies are returning to normal.
The TW, KT and W postcodes have been affected.
Thirty schools and two children’s centres in 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 has set up bottled water stations at a public car park near the Hampton Pub in The Avenue and opposite the Millennium Boat House in Lower Sunbury Road.
The company said that it expected “all [water] supplies to come back on over the course of the evening” following the repair work carried out.
A statement added: “We’ve delivered hundreds of bottles of water to customers on our priority services list, including those with medical and mobility issues.
“We’re really sorry for the inconvenience we’ve caused today and the time taken to resolve the problem.
“We’ll be carrying out a full investigation into what caused the burst so we can take steps to stop it happening again.”
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.