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.
A Thames Water statement 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.”