Hot summer days will now have to reach 28C (82.4F) in parts of England to be declared a heatwave.
The Met Office is increasing the temperature threshold across eight English counties as the climate warms.
A heatwave is classed relative to the current climate, and average temperatures are creeping up as the world gets hotter.
The three-day temperature threshold for a heatwave is jumping from 27C (80.6F) to 28C in six counties: Surrey, Berkshire, Buckinghamshire, Bedfordshire, Hertfordshire, Cambridgeshire.
In Lincolnshire, temperatures will have to reach 27C instead of 26C (78.8F) for three days to be determined a heatwave, and the East Riding of Yorkshire now has a 26C threshold, up from 25C (77F).
The weather body said data revealed an “undeniable warming trend for the UK”.
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The greatest temperature rise of more than 1C (1.8F) has been recorded across parts of central and eastern England. Further north, areas of Scotland and Northern Ireland have seen the mercury rise by around 0.7C.
“Although heatwaves are extreme weather events, research shows that climate change is making these events more likely,” said Dr Mark McCarthy, head of the Met Office National Climate Information Centre.
More frequent heatwaves
A Met Office study into the Summer 2018 heatwave in the UK showed that it was 30 times more likely to occur now than in 1750 because of the higher concentration of carbon dioxide, a climate-heating greenhouse gas, in the atmosphere.
As these concentrations increase, heatwaves of similar intensity “are projected to become even more frequent, perhaps occurring as regularly as every other year,” added Dr McCarthy.
The weather body calculates heatwave thresholds from historical data, previously on the period 1981-2010. The new limits are based on data from 1991-2020.
Heatwave thresholds are different to extreme heat warnings, which highlight very high temperatures to help protect lives and property.
The UK Health Security Agency heat-health alert for England warns about the impact of long periods of heat on human health.
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