People are being advised to stay indoors in the middle of the day and drink plenty of fluids.
Health warnings have been issued across the UK as temperatures are predicted to top 33C.
Level three heat-health alerts are in place across the south, the Midlands and eastern parts of England where temperatures are soaring.
The Met Office is advising people to stay indoors over lunch and to drink plenty of fluid to cope with the heat.
Parents are are also being encouraged to limit their children’s exposure to the sun.
The heat-health alerts are expected to stay in place until next weekend.
Met Office forecaster Greg Dewhurst said it could possibly be “the hottest day of the year so far” on Monday.
Heatwaves are becoming more likely and more extreme because of climate change.
Mr Dewhurst said central, southern and eastern parts of England could “possibly see maximum highs of 33C”, but for most it will be dry and sunny – and “well into the high 20s”.
The forecaster also warned temperatures could remain high overnight going into Tuesday – “remaining in the low 20s in cities, so many may experience an uncomfortable night”.
The UK’s highest temperature so far this year was 32.7C – recorded at London’s Heathrow Airport on 17 June.
Scotland and Northern Ireland had their hottest days of the year so far on Sunday.
Aboyne in the Highlands hit highs of 27.3C, while Derrylin in Country Fermanagh reached 24.3C.
Sweltering temperatures are also affecting the north of England, with Manchester set to reach a high of 28C by mid-afternoon.
Dr Agostinho Sousa from the UK Health Security Agency (UKHSA) has urged people stay hydrated and try to find shade when the rays are strongest between noon and 15:00 BST.
The agency said the elderly, people with underlying health conditions and those who live alone are particularly at risk.
It suggests people shade or cover windows exposed to direct sunlight, check fans and fridges are working properly, and that medicines are correctly stored.
The Met Office declares a heatwave when it records at least three days in a row with maximum temperatures exceeding a set temperature – which varies in different areas of the country.
The UK’s four-level heat-health system highlights the potential health impacts of high temperatures.
Rail passengers in the West Midlands are facing disruption to some services, with an operator blaming hot tracks.
High track temperatures meant fewer trains could run because of a speed restriction, West Midlands Railway said.
Some weather models are predicting extreme heat for the UK next weekend, but BBC weather presenter Sarah Keith-Lucas says it is much too early to call.
In some cases, temperatures above 40C (104F) are being predicted in southern England.
This is “the first time we’re ever seen UK temperatures above 40 appear in computer output, even if it is only a very tiny possibility”, she said.
Some weather models are showing extreme heat for the UK from next weekend.☀️🌡️ #UKWeather
But it’s much too early to call.
Here’s Sarah Keith-Lucas to explain why👇 pic.twitter.com/Y2MeywMwTE— BBC Weather (@bbcweather)
The world has already warmed by about 1.1C since the industrial era began in the latter half of the 18th century, and temperatures will keep rising unless governments around the world make steep cuts to emissions.
In England, there were 2,500 excess deaths in the summer of 2020 as a result of hot weather, while heat-related deaths in the UK could treble in 30 years, the British Red Cross predicts.
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