As the UK braces for temperatures toppling over 40C, fears are growing for the elderly and vulnerable.
A stark, public service announcement from the chief executive of the Met Office says the temperatures are "absolutely unprecedented" and warns Britons to stay inside.
Scorching temperatures are predicted for Monday, with Peterborough expected to hit 37C (99F) and Milton Keynes, Norwich and Lincoln set to see 36C (97F) - while temperatures could rise to 40C (104F) in London on Tuesday.
Penny Endersby, boss of the Met Office, said in her broadcast: "The extreme heat we are forecasting right now is absolutely unprecedented.
"Here in the UK, we are used to treating a hot spell as a chance to go and play in the sun. This is not that sort of weather. Our lifestyles and infrastructure are not adapted to what is coming.
"Please treat the warnings we are putting out as seriously as you would a red or amber warning for wind or snow, and follow the advice."
Her warning was echoed by College of Paramedics chief executive Tracy Nicholls, who said the "ferocious heat" the UK is predicted to experience over the next few days could lead to deaths.
She told Sky News's Sophy Ridge On Sunday programme: "This isn't like a lovely hot day where we can put a bit of sunscreen on, go out and enjoy a swim and a meal outside.
"This is serious heat that could actually, ultimately, end in people's deaths because it is so ferocious. We're just not set up for that sort of heat in this country."
The Met Office has issued an amber warning for the majority of England, which extends to southern Scotland and Wales from Monday until Tuesday.
The UK's first red extreme heat warning has also been issued across a large part of England, from London to Manchester and York on Monday and Tuesday, with transport services expected to be disrupted on both days.
The UK Health Security Agency has further announced a heat health warning at level four, which is described as a "national emergency", and ministers have discussed the situation at an emergency weekend COBRA meeting.
Boris Johnson, however, did not attend as he prepares for a farewell party to mark the end of his premiership.
Read more: What a Level 4 heatwave means for the UK
Climate attribution scientist at the Met Office Dr Nikos Christidis has said Tuesday's 40C prediction is a result of climate change.
But Deputy Prime Minister Dominic Raab told Sky News that while people should take precautions ahead of the record-breaking temperatures, they should also be able to enjoy themselves.
"Obviously there is some common-sense practical advice we are talking about - stay hydrated, stay out of the sun at the hottest times, wear sun cream - those sorts of things," he told Sophy Ridge On Sunday.
"We ought to enjoy the sunshine," he added. "And actually we ought to be resilient enough through some of the pressures it will place."
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Hospitals to be 'really, really pushed'
Additional contingency support for ambulance services, such as more call handlers and extra working hours, have been put in place on Monday and Tuesday.
The chairman of the NHS Confederation said hospitals are going to be "really, really pushed".
Lord Victor Adebowale said the NHS "will cope", but added that "coping isn't good enough".
Meanwhile, the British Veterinary Association warned dog owners to consider the impact rising temperatures could have on their pets.
Read more: How to keep your dog cool
A statement said at the mild end of the heat-related illness spectrum, a dog may feel tired, restless, struggle to sleep, lose its appetite, and pant consistently to try to lower its body temperature.
But at the more serious end of the spectrum (heat stroke) "your dog could lose consciousness, start fitting, pass bloody diarrhoea or vomit, bleed under the skin and eventually experience internal organ failure".
The association said that one in seven dogs in the UK die from conditions relating to the heat, with flat-faced breeds at particular risk due to their limited capacity to cool through panting.
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Greater Manchester Police implored people to avoid cooling off in reservoirs, rivers, or ponds after a teenage boy died while swimming with friends in a canal.
The 16-year-old got into difficulties and was last seen struggling in the water at Salford Quays in Greater Manchester at around 6.15pm on Saturday.
A boy's body was recovered overnight, and officers believe it may be that of the missing teenager.