Scotland and Northern Ireland had their hottest days of the year so far on Sunday - as other parts of the UK were hotter than Hawaii.
There were highs of 29.5C in London and the South East, with Wisley in Surrey recording 29.5C, and temperatures in the high 20s across most of England.
In Scotland the mercury hit 27.3C (81.1F) in the village of Aboyne on the edge of the Highlands in Aberdeenshire.
Derrylin, a village in County Fermanagh, reached 24.3C today - the highest of the year so far for Northern Ireland.
The heatwave is likely to last into next week, with temperatures set to soar above 32C (89.6F) on Monday and Tuesday, before dipping then rising again into next weekend.
An area stretching from London to south Yorkshire could see temperatures climb to 33C, which would be a new record for this year. The highest UK temperature for the year so far is 32.7C.
"Record breaking heat is possible" next weekend, Sky News weather producer Joanna Robinson said.
"At the moment, temperatures are likely to reach the low to mid-thirties in the South next weekend, but there is currently 30% chance the UK maximum temperature record will be beaten, which stands at 38.7C (102F)."
She warned: "There is now a worrying 10% chance that 40C (104F) will be reached for the first time in the UK."
The UK has already become about 0.9C hotter due to global warming driven by human activity, and summers are projected to become hotter still and likely drier.
By 2050, heatwaves like that seen in 2018 are expected to happen every other year.
Met Office forecaster Dan Stroud told the PA news agency: "It's been very warm across England today, temperatures staying in the high 20s.
The five-day forecast where you live
"Interestingly Scotland saw its hottest day of the year so far, while parts of Wales reached the mid-20s," said Mr Stroud.
"Northern Ireland saw the coolest conditions, reaching highs of 23.4C, but most of the UK was still warmer than Mediterranean islands like Sardinia and parts of Turkey," he added.
It comes after a heat-health alert was issued for England, 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.
While there is cooler weather on the way - it might not be for long.
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