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The UK could hit its highest-ever temperature today or tomorrow, surpassing the current record of 38.7C (101F) which was set in Cambridge in 2019.
The mercury may soar above 38C (100F), 39C (102F) or even 40C (104F) in some parts of the country, with the current heatwave due to peak on Tuesday, before cooler conditions push in from the Atlantic, rather than hot weather coming from southern Europe.
Scientists say climate change is increasing the likelihood of exceptional heatwaves in Britain, a country unaccustomed to such high temperatures.
So, how much are these kind of extreme heatwaves down to climate change?
Alex Deakin from the Met Office is in no doubt.
The meteorologist told Sky News: "We've had hot spells in the past. But what is absolutely clear is that these hot spells, these heatwaves are becoming more intense, more frequent. The science is absolutely clear that climate change has its fingerprints all over this current hot spell."
He added: "What we know is that 40C is now 10 times more likely in the UK than it would be under a naturally-varying climate - so one that humans haven't influenced.
"So we are going to experience these kind of heatwaves more and more frequently."
He went on: "Heatwaves are getting more severe, getting more ferocious, they are lasting longer and temperatures are rising and that's all part of climate change."
Jim Dale, senior meteorologist at British Weather Services, said the UK is at a "crossroads" in terms of the "most indisputable change sign that we've ever seen".
"Data has been one-way traffic for the last decade and more. The global records have all been in the last decade in terms of the heat and the CO2 levels.
"It's no longer denial - man-made climate change - it's here."
He added: "It's not just about heat. It's about other aspects as well that we'll see into the future - storms, floods - these aspects that climate change brings."
How ready is the UK to deal with such high temperatures?
Mr Deakin said the infrastructure in the UK is not ready to cope with such levels but it must increasingly prepare for them, including by the government.
He called them "unprecedented values", adding: "When you go on holiday your body is prepared for it and the infrastructure in those countries (such as southern Europe) is set up. You have air conditioning."
Mr Deakin warned: "It's the night-time temperatures, as well as the daytime temperatures, that are going to cause the problems (in the UK) as people won't be able to sleep as well, and if your body does not get that rest then it really struggles to cope in the high temperatures."
Met Office chief executive Penny Endersby said people can find it hard to know what to expect when "climate change has driven such unprecedented severe weather events".
"Here in the UK we're used to treating a hot spell as a chance to go and play in the sun," she added.
"This is not that sort of weather."
What should the UK do?
Nigel Arnell, a professor of climate system science at the University of Reading, said Britain needs to prepare for more hot weather in the future, retrofitting buildings to cope with extreme weather and planting more greenery in cities.
Adaptation and resilience need to become a political priority, he said.
"We can't keep on dealing with extremes in crisis mode," Mr Arnell said.
Susan Scholefield - a former director of the Cabinet Office's Civil Contingencies Secretariat - said people should make their homes more fuel efficient.
In the immediate situation, she said people should adapt their behaviour.
"Close curtains during the day and open them up during the night," she suggested.
London mayor Sadiq Khan has urged Londoners to only travel on Monday and Tuesday if it is essential and to prepare for disruption because speed restrictions will be in place on rail and Tube networks.
And fire brigades, including South Wales Fire And Rescue Service, Scottish Fire And Rescue and London Fire Brigade, have issued safety warnings, urging people to act responsibly.
They warn people to dispose of barbecues, lit cigarettes and glass bottles responsibly, to not burn any rubbish such as garden waste and use local authority services instead.
And that barbecues should not be used on balconies or near sheds, fences, trees, shrubs and garden waste to avoid anything catching alight.
They also urge people who are cooling off in waterways to be aware of cold-water shock.
What is the evidence from the Met Office linking climate change to UK heatwaves?
The Met Office looked at the summer of 2018, the joint warmest on record.
It found the chance of such a hot summer in a natural climate was just 0.5%. But because of greenhouse gases in the atmosphere, that had increased to 12%.
In other words, a record summer is almost 30 times more likely now as a result of climate change.
The warming trend will accelerate. By mid-century, the Met Office predicts a summer as hot as 2018 will happen every other year.
And if greenhouse gas emissions continue to climb as predicted, by the end of the century UK temperatures will peak at 40C (104F) or more once every three to four years.
How many heatwave-related deaths are there annually in the UK?
Such sweltering heat is life-threatening - particularly for the young and old - when temperatures remain high for several days.
Heatwave deaths currently average around 2,000 a year. By 2050, they're predicted to reach 7,000.
"Even as a climate scientist who studies this stuff, this is scary," said Professor Hannah Cloke, a natural hazards researcher at the University of Reading, about the current situation.
"This feels real. At the start of the week I was worried about my goldfish getting too hot. Now I'm worried about the survival of my family and my neighbours."
What will weather experts learn from this latest heatwave?
Sky weather producer Kirsty McCabe said scientists wait until after the event, once we see how hot it actually got, before they can attribute it to climate change.
She said: "They compare what temperatures we would expect in different scenarios (ie different amounts of carbon dioxide in the atmosphere). With the extreme temperatures linked to higher emissions."
She added: "Current research says heatwaves will happen more frequently, last longer and be more intense, unless we curb global warming. If not, 40C in the UK could occur every few years in the future."
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