Parts of the UK could see temperatures pushing towards 30C (86F) today, according to forecasts.
London and southeast England are expected to see highs of 29C (84.2F), with clear skies during the afternoon.
The heatwave is likely to last into next week, with temperatures set to soar above 32C (89.6F) on Tuesday, which would make it the warmest day of the year so far.
Central England is likely to reach 28C (82.4F), with the South West hitting 23C (73.4F).
In Wales, the mercury is forecast to reach 26C (78.8F) on Sunday afternoon, northern England could see 27C (80.6F), Scotland could reach 25C (77F) and Northern Ireland could get to 23C.
Met Office meteorologist Dan Stroud: “There’s a headline maximum of around 29C over the course of Sunday in London and the South East.
“We should see pretty much wall-to-wall sunshine across the bulk of England and Wales and a good portion of Scotland.”
It comes after a heat-health alert was issued for most of the country, with people warned not to stay in the sun for long periods and to make sure they are covered in high-factor sun cream.
The UK Health Security Agency (UKHSA) and the Met Office have put in place a level 3 alert for the East of England, South East and London with highs of 32C (89F) currently forecast. A level 2 alert has been issued for the South West, East Midlands, West Midlands, North West and Yorkshire and the Humber regions.
A level 2 alert is issued when there is a “high chance” of the forecasted conditions, while level 3 means there is a “90% probability”. The alerts will be in place from 9am on Monday until 9am on Friday.
The hottest day in the UK so far this year was 17 June, when Heathrow reached 32.7C, the Met Office said.
Temperatures are expected to cool later this week, as a cold front pushes in.
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