Thousands of people in East London and many in Reading, Berkshire are without water in their homes or under water pressure at home.
In nine London postcodes and readings, two customers reported not washing their hands at the risk of the coronavirus.
Thames Water said engineers are investigating and will do their best to get supplies back as soon as possible.
The outage, which was supposed to start at around 4.30pm, damaged homes on the postcodes E6, E7, E10, E11, E12, E13, E15, E16 and E20, as well as the RG30 and RG31.
Some people in London started buying bottled water for hand washing, while others complained that they could not go to Thames Water on the phone.
To teach, Thames Water was giving bottled water. “Please consider and do not take more than you need,” it was posted on its website.
The water chief apologized for the leak and added: “We know how disturbing and inconvenient it is.
“We are experiencing a high number of calls at the moment. Call only in case of emergency. Rest assured, we are trying our best to solve the problem. “
Some social media users advise others to collect rainwater in buckets.
Others offer to sell bottled water. One user said he could supply 500ml bottles for 10.99ml.
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