Thames Water, which supplies 15 million people, has put in place a hosepipe ban which comes into effect today.
The company said water levels in its reservoirs were "much lower than usual" when it announced the ban.
It joins Welsh Water, Southern Water and South East Water and South West Water in implementing bans, with Yorkshire Water following suit on August 26 - meaning more than 29.4 million customers across the UK will be prohibited from using hosepipes by the end of the week.
It comes after a drought was officially declared across most of England following the driest July for 50 years and the driest first half of the year since 1976.
Parts of the UK faced two days of heavy downpours and flooding last week, but the Environment Agency has said it will take weeks' worth of rain to replenish water sources and end the drought.
In a statement on its website, Thames Water said: "We have more teams reducing leakage than ever before, working 24/7 to find and fix more than 1,100 leaks every week. The recent heatwaves mean that demand for water is also at record levels.
"We've been working around the clock to supply everyone, and customers have been brilliant at saving water where they can. But, with low rainfall forecast for the coming months, we now need to take the next step in our drought plan. Everything we do now will help protect supplies next summer and help the environment.
"We know these restrictions impact your day-to-day activities around your home and beyond, and we're grateful for your support."
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