Thames Water has ordered Britons to take "shorter showers" as stocks dwindle amid ongoing drought.
The London utility, which supplies water to 16 million customers, has urged Britons to reduce water usage as it revealed water reservoir levels were lower than expected for this time of the year.
The utility said its London reservoir was down to 74 per cent, below an average of 87 per cent for this time of year.
Its Farmoor Reservoir in Oxfordshire was 92 per cent full, but this was also slightly below the average of 96.4 per cent for this time of year and was steadily declining.
Martin Padley, water director at Thames Water, said water reduction measures taken by customers had been "making a real difference" to protect water supplies, as demand dropped by three per cent across London and more than six per cent.
"With reservoir levels below where we would expect them to be for this time of year and demand remaining high, we're asking customers to keep going and continue using only what they need," he said.
"Small changes, such as taking shorter showers, turning off taps and avoiding non-essential water use outdoors, all help reduce pressure on our network while our teams continue working around the clock to find and fix leaks."
Rainfall in the Thames catchment area during July was less than 1 millimetre, just two per cent of the long-term average, Thames Water said.
Customers had used about 100 million litres more water every day than normal, putting more pressure on rivers, reservoirs and the water network, the company added.
Britain will see its "first meaningful rain for a long time" this week on Wednesday, according to the Met Office.
But the showers are unlikely to end the drought, with the forecaster warning the rain would not be a "huge amount" and "highly hit and miss from place to place".
In an online forecast, Met Office meteorologist Aidan McGivern said Wednesday would be a "very showery day" across the UK, with heavy downpours and frequent showers in Northern Ireland and some parts of western and central Scotland.
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Showers in southern areas would be "fewer and further between", while central England and eastern Wales could see some downpours.
He said: "I suspect many people across England and Wales will at least enjoy the feel of some rain falling out of the sky during the next few days, but it’s not going to solve the drought situation overnight because we’re not expecting huge amounts and it will be highly variable because a lot of it will come in the form of showers."
On Wednesday, highs of 25C in London, 20C in Cardiff, 19C in Manchester and 16C in Glasgow are expected, according to the weather service.
And Thursday will kick off with some "very heavy rain" across parts of northern England and north and west Wales.
That afternoon will bring a "better chance" of showers further south and "dumping a reasonable amount of rain in places", Mr McGivern said.
Wessex Water introduced a hosepipe ban for its 1.4 million customers for the first time in 50 years.
Customers across areas in Bath, Somerset, Dorset and Wiltshire will have water restricted as the drought significantly impacted the company's underground storage reservoirs.
Hosepipe bans are now in place for more than 27 million people across England.
The hottest day of the year was recorded in Kew Gardens, London, on Thursday when temperature reached 38.1C.
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