People are being encouraged to look out for vulnerable and elderly people as temperatures are predicted to hit highs of 33C (91F) in parts of the UK – warmer than Athens and Ibiza.
Met Office forecaster Greg Dewhurst said it could possibly be “the hottest day of the year so far” on Monday.
The alerts will be activated at 09:00 BST and remain in place until Friday.
The east and south of England will see an amber level three alert, with the rest of England on a level two alert.
Heatwaves are becoming more likely and more extreme because of climate change.
As temperatures rose over the weekend, parents were advised to limit their children’s exposure to sun.
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 from the morning”.
“So it’s very possible tomorrow will be the hottest day of the year so far, it will certainly be on par with the existing record, maybe more.”
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”.
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 11:00 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.
A 2022 high of 28.5C was recorded in St James’ Park in London on Friday.
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.
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) July 10, 2022
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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