Up to 40,000 homes are believed to have been left without water after a failure at a supply plant in the South East.
Customers across West Sussex were left with no water supply at the weekend after low levels of chlorine at a water supply works prompted an outage.
Southern Water said on Saturday night that the issue at the works in Hardham had left customers whose water came from reservoirs in Upper Nash and Itchenfield with no water or low pressure.
The issue meant tens of thousands of homes woke up to no water on Sunday, with the company saying that 20,000 customers were affected - with the number expected to grow to 40,000 throughout the day.
In updates on its website, Southern Water said bottled water stations had been opened and also announced that it would be delivering water to vulnerable customers.
On Sunday it said it had been delivering to its most vulnerable customers, and said a fleet of 19 tankers was supporting hospitals and supply where most needed.
In an update on Sunday morning, the firm said: "We’re sorry that customers in areas of West Sussex are waking up to no water.
"Our sites operate to the strictest standards for water quality and are designed to automatically shut down well in advance of any risk to our customers.
Emergency water just turning up at Toat Cafe on A29.
I have raised with Southern Water in a meeting this morning – and they agreed – that we urgently need a bottled water station south of the Church Hill one-way system on the A29 at Pulborough to serve residents in the south of… pic.twitter.com/PqiFZ8BHXJ
— Andrew Griffith MP (@griffitha) May 14, 2023
"The incident was caused by unusually low levels of chlorine being detected at our Hardham water supply works. Chlorine is an important part of our standard treatment processes used to ensure water quality meets the very high standards set.
"Our teams are investigating what has caused the change in chlorine levels. We can confirm this has nothing to do with sewage."
In a further update, it said it was unable to supply water to approximately 20,000 customers in West Sussex, adding: "We anticipate that this number will grow during the morning, and could reach 40,000 at around lunchtime.
"We're doing everything we can to fix this and return the plant to service."
On Sunday afternoon, Southern Water said it had made "good progress" and had stabilised chlorine levels, so Hardham Water Supply Works was operational again.
It said: "Our next steps are to refill the reservoirs and recharge the network. We need to take a cautious and gradual approach, and this activity will be ongoing throughout the afternoon and into the evening.
"You might experience intermittent water supply as the recharge takes place, this is totally normal. Please don’t be concerned therefore if your water returns and then you lose it again during this period.
"Providing our plans progress as anticipated, customers should see their water return in the next 24 hours."
Andrew Griffith, Arundel and South Downs MP, posted regular updates on his Twitter account, including from visits to various water stations.
He wrote: "Water is one of the few absolute essentials of life. Everyone understands that things can occasionally go wrong.
"But at times like these the mark of a good organisation is excellent communications and a rapid response to fix things or to help those most impacted.
"We need our local Councils and Southern Water to step up and help households who have already been without even a basic water supply for 12 hours."