Hospital appointments have been cancelled and over 20 schools closed after a burst pipe left thousands of people in Richmond with little or no water.
Thames Water said the fault at its main regional pumping station at 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.
Thames Water tweeted: "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 are there investigating and we have more experts planning how to get water back on for our customers as soon as we possibly can.
"If we're unable to do that within the next few hours we'll make bottled water available across the affected areas and we encourage customers to keep an eye our feed for the latest updates."
Richmond Council said a number of schools had closed, as well as Teddington Pools, Orleans House Gallery and Hampton Hill Library.
All planned clinics and sessions at Teddington Memorial Hospital, including the Urgent Treatment Centre, and Teddington Health and Social Care Centre were cancelled.
A number of GP services have also closed.
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.
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".
Meanwhile, Twitter users posted pictures of supermarket shelves stripped of bottled water.
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.
For up to date info, visit: https://community.thameswater.co.uk/t5/ ... W/td-p/532
Photo: The water aisle in a Waitrose supermarket in Twickenham, as a burst pipe left large parts of London with little or no water (David Mirzoeff/PA)