By early afternoon (late Monday night AEST), the mercury sat at 35C in London. The highest temperature ever recorded in Britain is 38.7C, a record set in 2019.
While Monday is likely to bring record highs to southeastern England, temperatures are expected to rise further as the warm air moves north on Tuesday, Met Office CEO Penelope Endersby said.
The extreme heat warning stretches from London in the south to Manchester and Leeds in the north.
“So it’s tomorrow that we’re really seeing the higher chance of 40 degrees and temperatures above that,” Endersby told the BBC.
“Forty-one isn’t off the cards. We’ve even got some 43s in the model, but we’re hoping it won’t be as high as that.”
Hot weather has gripped southern Europe since last week, triggering wildfires in Spain, Portugal and France. Almost 600 heat-related deaths have been reported in Spain and Portugal, where temperatures reached 47C last week.
Climate experts warn that global warming has increased the frequency of extreme weather events, with studies showing that the likelihood of temperatures in the UK reaching 40C is now 10 times higher than in the pre-industrial era.
Drought and heat waves tied to climate change have also made wildfires harder to fight.
Officials in southern France’s Gironde region announced plans to evacuate an additional 3500 people from towns threatened by the raging flames.
More than 1500 firefighters and water-bombing planes are trying to douse the flames in the region’s tinder-dry pine forests.
In Britain, train operators are asking customers not to travel unless absolutely necessary because the heat is likely to warp rails and disrupt power supplies, leading to severe delays.
Some medical appointments have been canceled to relieve strain on the health service. While some schools have closed, others are setting up wading pools and water sprays to help children cool off.
Britain is unaccustomed to the temperatures forecast this week, and few homes, schools or small businesses have air conditioning. Across the UK, average July temperatures range from a daily high of 21C to a low of 12C.
Nightfall will bring little relief from the heat, with the Met Office forecasting temperatures of 29C at midnight in London.
Monday night will be “very oppressive” and it will be difficult to sleep, Chief Meteorologist Paul Davies said.
“Tomorrow is the day where we are really concerned about a good chance now of hitting 40 or 41C, and with that all the health conditions that come with those higher temperatures,” he said.