Schools have been closed and hospital appointments cancelled due to a burst pipe that has left large parts of 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 wanted to get people’s water back on “as soon as possible” but bottled water would be made available if the problem was not fixed soon.
The TW, KT and W postcodes have been affected.
Trafalgar Junior School in Twickenham, which has been left without flushing toilets and washing facilities in the kitchen, has sent children home.
Radnor House School in Twickenham also closed at 10:30 BST but exams were still taking place.
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 because of the problem.
The racecourse tweeted there was “no estimated time of the water supply being restored today”.
Thames Water said: “We’re sorry to anyone whose water supply has been affected by a burst on a large pipe at our water works in Hampton.
“A team of engineers is there investigating and we have more experts planning how to get water back on for our customers as soon as we possibly can.”
Mayor of London Sadiq Khan said it was “unacceptable that so many people are 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.
Tanya O’Connell, who lives in Twickenham, said the lack of water 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.