A hosepipe and sprinkler ban has come into force for South East Water customers in Kent and Sussex.
The measure affects at least one million people, and rule-breakers could be hit with a £1,000 fine.
The ban, imposed from 00:01 BST on Friday, means that using hosepipes to water gardens, clean cars and fill swimming pools will not be allowed.
South East Water said it "had no choice" following the driest July on record.
It is the second water company in England to implement a ban this year, following Southern Water which brought in restrictions in Hampshire and the Isle of Wight last Friday.
Thames Water this week announced it would be bringing in a hosepipe ban "in the coming weeks".
It comes after the Met Office issued a four-day amber extreme heat warning, meaning vulnerable people's health could be impacted and travel could be disrupted.
There are some exceptions to South East Water's ban, including people who are disabled, those with commercial vehicles and fish-pond owners.
The firm said it was "relying on people's goodwill" to observe the ban, but it urged customers to contact them if they spot a neighbour ignoring the restrictions.
Following weeks of disruption to water supplies for some towns and villages in Kent and Sussex, the water firm issued pleas to customers to be mindful of how much they use during the hot weather.
A number of burst water mains and leaks also caused problems to supplies.
A spokesperson for the water company said: "We have been producing an additional 120 million litres of water a day to supply our customers, which is the equivalent of supplying a further four towns the size of Maidstone or Eastbourne, daily.
"We have been left with no choice but to restrict the use of hosepipes and sprinklers."
It added the ban would stay in place "until further notice".
If you live in Sussex and Surrey you can keep up to date on which restrictions apply in your area, tips to save water, and the latest on how the counties are coping with the dry weather by listening to updates on BBC Radio Sussex and Surrey's "Hose Down Lowdown" on BBC Sounds.
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