The Met Office is increasing the temperature threshold for heatwaves by a degree across eight English counties due to climate change.
Heatwaves are defined as happening when a location records at least three days in a row with daily maximum temperatures meeting or exceeding a certain threshold.
The weather and climate body said the three-day temperature threshold for a heatwave would increase from 27C to 28C for six counties: Surrey, Berkshire, Buckinghamshire, Bedfordshire, Hertfordshire, Cambridgeshire.
In Lincolnshire, temperatures have to reach 27C for three days for it to be classed as a heatwave, an increase from 26C, and the East Riding of Yorkshire now has a 26C threshold, up from 25C.
The heatwave definitions are designed to be relative to the current climate, and with average temperatures rising in the UK as a result of global warming, the thresholds are being increased for parts of England.
Dr Mark McCarthy, head of the Met Office National Climate Information Centre, which manages the UK's climate records, said: "Climate statistics over time reveal an undeniable warming trend for the UK.
"Temperature rise has been greatest across parts of central and eastern England where they have increased by more than 1.0C in some locations, while further north areas of Scotland and Northern Ireland have seen temperatures rise by closer to 0.7C."
And he said: "Although heatwaves are extreme weather events, research shows that climate change is making these events more likely.
"A scientific study by the Met Office 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 greenhouse gas, in the atmosphere.
"As greenhouse gas concentrations increase, heatwaves of similar intensity are projected to become even more frequent, perhaps occurring as regularly as every other year."
Previous heatwave thresholds were based on the climate in the period 1981-2010, and were intended to be reviewed and revised to represent heatwaves relative to the current climate, the Met Office said.
The new thresholds are based on the climate from 1991-2020, which was brought in as the reference period in January this year, and are being updated ahead of summer.
The heatwave thresholds are separate from extreme heat warnings, which highlight very high temperatures to help protect lives and property, and the UK Health Security Agency heat-health alert, issued in England only, to flag up the impact of prolonged heat on public health.
