Thames Water is introducing a hosepipe ban across Swindon, Gloucestershire, Oxfordshire, Berkshire and Wiltshire.
It comes after the Environment Agency placed the Thames area into its Prolonged Dry Weather category.
The Temporary Use Ban, in areas with a postcode starting with OX, GL, SN and RG4, RG8 and RG9, will come into operation at one minute past midnight on Tuesday 22 July.
Customers living in areas affected by the ban are asked not to use hosepipes, including for cleaning cars, watering gardens or allotments, filling paddling pools and swimming pools or cleaning windows.
It follows the driest spring in over a century and England's warmest June on record.
Nevil Muncaster, strategic water resources director at Thames Water, said: "This has been a challenging spring and summer with big spikes in customer demand during hot dry days and very little rainfall to replenish local supplies in the Thames Valley.
"Given the continued warm, dry weather we do not anticipate that the situation will improve any time soon so we have to take action now. Every drop of water we use comes from the environment and we have to balance protecting this while taking out the water needed to top-up reservoir levels.
"That is why we have had to make to the difficult decision to implement a hosepipe ban to make sure there's enough water for customers' essential needs and to protect rivers.
"While this TUB will only affect customers in certain areas, I want to thank all our customers for their understanding and for their support so far to reduce their water use. This includes turning the taps off while brushing teeth, taking shorter showers and letting your lawn go dry."
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