Thousands of people were left without water on Sunday due to a problem at a Thames Water treatment plant.
Thames Water has apologized to residents in Godalming, Surrey, and said it was investigating.
Jeremy Hunt, MP for the area, said a major incident had been declared. He posted on X, formerly known as Twitter, that he was very concerned and had spoken to the CEO of Thames Water.
It follows problems caused by Storm Ciaran at Shalford water treatment plants, Thames Water said.
Mr Hunt said the CEO told him at 2.50pm that 13,500 customers had no water and another 6,500 were expected to lose their supply soon.
Site controls have been reset and are being tested, he added.
“If this is successful, they will be able to increase production to a level that will restore supply,” he said.
Surrey County Council confirmed a major incident had been declared in the area, telling Sky News it was “due to the number of properties affected” and also the fact the incident occurred on Sunday, when there are fewer staff.
Flood warnings remain after Storm Ciaran – but rain expected to pass
Bottled water stations have been installed at the Crown Court car park in Godalming and Artington Park and Ride in Guildford.
Thames Water said it was also delivering bottled water to vulnerable customers.
Thames Water said its engineers were on site and tankers were being used to pump water into its supply network.
A Thames Water spokesperson said: “We are sorry to those who have no water or have lower than normal pressure.
“(We) are doing everything we can to get things back to normal as quickly as possible.”