Thames Water has announced a hosepipe ban for 10 million customers across the south of England.
The ban will come into force from 24 August and will impact people across the Thames Valley and London.
The temporary use ban comes after reports of the River Thames reaching its lowest level since 2005 and "unprecedented weather conditions".
It means people will not be able to use hosepipes to water gardens, wash cars and windows, or fill paddling pools.
Thames Water CEO Sarah Bentley said implementing the ban had been a "very difficult decision" but, she added, reservoir storage levels in London and Farmoor, in Oxfordshire had "reduced significantly".
The ban is the next phase in the water firm's drought plan, having already announced it will fix 1,100 leaks across its 20,000-mile network every week.
Ms Bentley added: "After months of below average rainfall and the recent extreme temperatures in July and August, water resources in our region are depleted.
"Customer demand is at unprecedented levels and we now have to move into the next phase of our drought plan to conserve water, mitigate further risk and futureproof supplies."
Thames Water admits it must do better as ban loomsLondon officially declared in droughtDriest first half of year in England since 1976
Thames Water, which also has five million wastewater customers, is the latest water provider to announce a hosepipe ban as droughts have been declared across England.
The firm previously admitted it "must do better" after coming under fire for having the worst record on leaks out of all nine water companies in the UK.
More than 600m litres of water is currently lost a week by the company, mainly due to ageing pipes.
Hosepipe bans come after temperatures topped 40C in the UK for the first time on record this summer.
The recent heatwave resulted in the highest demand for water in the past 25 years, according to Thames Water.
The company said it supplied 2.9 billion litres of water a day to customers across the region as temperatures spiked.
In areas with particularly hot weather, the firm said demand rose by 50% compared to the average for the time of year.
Taps in the Oxfordshire village of Northend ran dry in a heatwave last week and Thames Water had to deliver emergency supplies to 68 customers.
Resident Dr Gina Brown, a radiologist who lives in the village with her elderly parents, p told the BBC water shortages were a "recurrent" problem during hot weather.
It was later found the issue was down to E.coli being found in a reservoir which supplies the village.
Thames Water apologised for the disruption and said the site had been disinfected.