Thousands of people in east London and many in Reading, Berkshire, were left without water or suffering low water pressure in their homes.
Customers in nine postcodes of London and two in Reading said not being able to wash their hands was a coronavirus risk.
Thames Water said engineers were investigating and would do everything they could to get supplies returned as quickly as possible.
The outage, thought to have started at about 4.30pm, affected homes in postcodes E6, E7, E10, E11, E12, E13, E15, E16 and E20, as well as RG30 and RG31.
In London, some people started buying bottled water to wash their hands, and others complained they could not get through to Thames Water on the phone.
In Reading, Thames Water was giving out bottled water. “Please be considerate and don’t take more than you need,” it posted on its website.
Water chiefs apologised for the outage, adding: "We know how worrying and inconvenient this is.
“We are experiencing a high number of calls at the moment. Only call if it's an emergency. Rest assured, we're doing all we can to get the issue resolved.”
Some social-media users advised others to collect rainwater in buckets.
Others offered to sell bottled water. One user said he could deliver a 500ml bottle for £10.99.
