A hosepipe ban for Thames Water customers will start on 24 August due to "unprecedented weather conditions".
Although the ban officially starts next week, Thames Water urged customers to act as soon as possible to save water.
In a statement, it said: "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."
The ban will mean people must not use a hosepipe that is connected to the mains water supply - ruling out a number of jobs around the home which people might believe are exempt.
The ban is being brought in to all customers that get their water from Thames Water.
All around the UK water levels are struggling following record-breaking heat and historically low rainfall.
Anyone taken to court for persistent breaches of restrictions on hosepipe bans - including watering a garden, cleaning a vehicle, or washing windows, walls, paths, and patios - faces a fine of up to £1,000.
The announcement comes despite heavy rain and flooding hitting some parts of the UK and a yellow warning for thunderstorms issued.
A lack of rain and high temperatures have caused drought conditions which have turned much of the country's landscape from green to brown and yellow.
Significant fires were reported in parts of London, Kent, and Essex over the weekend, while the weather has also resulted in incidents of people getting into difficulty while swimming in lakes, rivers, and the sea.