Water bosses at a firm whose reservoirs are full have demanded Britons only fill their paddling pools to the halfway line.
South West Water, which loses 107 million litres a day through unrepaired leaks, told customers a half-filled pool was "just as much fun".
Killjoys urged customers to limit filling up paddling pools - despite its reservoirs being 90 per cent full.
The company, which covers Dorset, Somerset, Devon and Cornwall, said: "A half-filled pool is just as much fun. Cover your pool to keep the water usable for up to three days."
South West Water owns 23 reservoirs across Devon and Cornwall, with levels between 85 per cent and 90 per cent.
Liz Northcott, 74, told The Sun: "My great-grandchildren are in the paddling pool and there will be no half measures."
"They don't have a shortage, so why tell us to use less water?" she added.
The intervention came just as Britain marked its hottest May day on record, with temperatures reaching 35C in parts of London.
Harlyn Bay beach in Cornwall was the site of 231 spillage dumps
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The Met Office has confirmed conditions climbed to their highest at Heathrow Airport and Kew Gardens in the capital.
Monday had set a new record with 34.8C - again, at Kew Gardens.
The previous May peak had stood since 1922 when Camden Square experienced a high of 32.8C.
On the other side of the country, South East Water faced MPs' fury after taps ran dry over the weekend.
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The River Camel was the site of 336 recorded incidents of spillage between March 2013 to 2020
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More than a thousand customers across Kent and East Sussex were left high and dry across the sweltering Bank Holiday thanks to a series of infrastructure failures.
Dave Hinton, its former chief executive, resigned over his response to major outages that left thousands of customers without running water.
Chris Train, the company's former chairman, also stepped down earlier this month.
The supply crisis began in the run-up to Christmas, with approximately 24,000 properties across Kent and East Sussex experiencing disruptions during November and December.
Britons were told to stop filling their paddling pools amid the May heat
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Weeks later, up to 30,000 homes endured several days of water chaos.
Canterbury MP Rosie Duffield said: "Despite the recent dramatic change of management at South East Water, we have yet to see any change in acceptable levels of service or visible leadership.
"Yet again my constituents are left without water in a heatwave, the most basic necessity.
"We need to see proper investment, effective management and urgent intervention by Ofwat and the Government to ensure these incidents stop happening on a regular basis."
South West Water said: "We support our customers to use water wisely in all weathers."