More than two million people will be subject to a hosepipe ban amid hot weather.
South East Water said 2.4 million customers will be affected by temporary restrictions in Sussex, Surrey, Hampshire and Berkshire from Saturday.
But the firm is asking customers to cut back water use straight away, telling them not to use hosepipes for their gardens, cars, patios, boats or swimming pools.
The ban had been in place just for its customers in Kent.
The firm said it has seen a “decline in river levels” and needs to “protect some groundwater sources” after lower than average rainfall, hot weather and high demand for water.
Douglas Whitfield, water supply director at South East Water, said: “Our absolute priority has to be making sure there is always enough water for everyone’s essential daily needs: drinking, cooking and hygiene.
“To safeguard our shared resource and help prevent any homes from facing a sudden loss of water, we sadly need to ask our communities to not use their hosepipes immediately.
“We are deeply sorry for the disruption this causes, and we are incredibly grateful to everyone helping us protect water supplies in Sussex, Surrey, Hampshire and Berkshire.”
It comes as south-east England has seen the longest unbroken spell of no recorded rainfall for more than 30 years, Met Office figures suggest.
An average of zero rainfall has been measured across the organisation’s weather stations in the region for 17 days in a row, from July 2 to July 18, according to the latest available data.
This is the longest unbroken run of days with no rain for south-east England since a 17-day period in March-April 1995.
Data for July 19 has yet to be published, but could show the rain-free spell extending for even longer.
Reservoir storage levels in England are currently at 79%, around five percentage points below the long-term average for the time of year, the Environment Agency said as it released its weekly dry weather and drought report on Friday.
Six reservoirs are classed as having “notably low” levels, including Wimbleball in Somerset, Grafham in Cambridgeshire and Hanningfield in Essex, while Blagdon reservoir in Somerset is classed as being “exceptionally low”.
It comes after there have been 28 days in 2026, consecutive and non-consecutive, when 30C has been exceeded somewhere in the UK: seven in May, eight in June and 13 in July.
The record for the number of 30C-plus days in a calendar year is 34, which was set in 1995.
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