The chances of seeing 40C days in the UK could be as much as 10 times more likely in the current climate than under a natural climate unaffected by human influence.
The risk of days with sweltering 40C heat in the UK could rise significantly by the end of the century without action to drive down greenhouse gas emissions, the Met Office has warned.
Climate change caused by human activity is already pushing the mercury to new record highs, with 38.7C in Cambridge in July 2019 the highest temperature ever recorded in the UK.
Those new records prompt the question of whether 40C heat is on the horizon for the UK, with heatwaves posing a potentially severe risk to people's health.
Researchers at the Met Office Hadley Centre have used a detailed local-scale dataset based on observations to assess the likelihood of future hot spells in the face of high or medium levels of greenhouse gas emissions.
Climate change, driven by greenhouse gas emissions from activities such as burning fossil fuels which heat up the planet, has put the UK on a course to see extremes that would be highly unlikely under a "natural" climate, the scientists said.
Currently the chances of temperatures reaching 40C anywhere in the UK are extremely low.
But if emissions continue at high levels, worsening climate change, the UK could see days with 40C heat every three to four years on average by 2100, the study published in the journal Nature Communications found.
Temperatures exceeding 35C currently occur once every five years on average, but that could rise to every other year with high emissions.
The scientists said if the world takes action on emissions in line with commitments in the international Paris Agreement to limit temperature rises to 1.5C or 2C above pre-industrial levels, the risk of extreme heat would be much lower.
The south-east of the UK is more likely to see 40C temperatures, while 35C temperatures are becoming increasingly common in the region.
But the study also found that areas in the north, where it is extremely unlikely to see days that reach 30C, may exceed that temperature at least once a decade.
Dr Nikolaos Christidis, the lead author, said: "We found that the likelihood of extremely hot days in the UK has been increasing and will continue to do so during the course of the century, with the most extreme temperatures expected to be observed in the south-east of England.
"Climate change has already influenced the likelihood of temperature extremes in the UK.
"The chances of seeing 40C days in the UK could be as much as 10 times more likely in the current climate than under a natural climate unaffected by human influence.
"The likelihood of exceeding 40C anywhere in the UK in a given year has also been rapidly increasing, and, without curbing of greenhouse gas emissions, such extremes could be taking place every few years in the climate of 2100."
Co-author and head of the Met Office National Climate Information Centre Dr Mark McCarthy said: "This research shows human-caused climate change has set us on a course to see temperature extremes in the UK that would be highly unlikely under a 'natural' climate, although urgent action to reduce emissions now can significantly reduce the occurrence of extreme high temperatures in the UK in the future."
The government's advisory Committee on Climate Change has previously warned the UK is unprepared for the health impacts of increasing heatwaves, which can raise the risk of illness and deaths, particularly among the elderly or those with existing health conditions.
Action is needed to prevent overheating in homes, hospitals and on public transport, the committee has warned.
Emily Beament is the PA environment correspondent.