Thousands of homes in west and southwest London have been left with low pressure or without water, Thames Water says.
The firm said the problem had been caused by a power supply problem that hit its Ashford and Hampton water treatment works in west London.
The following postcodes are affected: CR4, E5, E8, EC1V, KT1, KT2, KT3, KT8, HA0, N1, N15, N16, N17, NW1, NW2, NW10, SE1, SE11, SE16, SE17, SW4, SW8, SW9, SW13, SW14, SW15, SW18, SW19, TW4, TW5, TW7, TW8, TW10, TW11, TW12, TW14, TW17, UB1, UB2, UB6, W1H, W1H, W1M, W1N, W2, W3, W4, W6, W7, W10, W12, W13, W14.
Thames Water said: “We’re working hard to fix the issue and to get things back to normal.
“We’ll provide further updates here as soon as we have more information.”
MP for Twickenham in southwest London, Munira Wilson, said some schools in her area have been forced to remain closed over the issue and urged Thames Water to “sort this soon”.