The UK is bracing for its fifth heatwave of the year, with possible highs of 36C (97F) on the way.
Temperatures will climb through the weekend before highs of 36C could be reached by the middle of next week.
Chris England, Sky News' meteorologist, said: "Temperatures are climbing again, with heatwave thresholds likely to be exceeded for an unprecedented fifth time this year.
"England looks hottest, with 36C possible on Wednesday and Thursday, with Wales only a little behind, but there's a chance that all four nations may go over the limits."
Parts of East Anglia and the South East can expect the hottest temperatures next week, according to the Met Office.
The UK has already seen two record-breaking heatwaves, in May and June, which have been linked to more than 2,700 excess deaths. Two further heatwaves in July have fuelled drought and wildfires.
Dr England, however, noted that because of how heatwaves are recorded, the recent heat "could well be described as part of a prolonged heatwave interspersed with a few brief cooler interludes".
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There have been 33 days so far this year when temperatures have exceeded 30C somewhere in the UK and the record of 34 days - set in 1995 - will likely be broken next week.
Yellow heat health alerts, which warn of possible impacts across health and social care services, came into force from 9am on Saturday and run to 9pm on Tuesday, covering most of England, apart from the North East and North West.
Forecasters say the weekend will see a national divide in the weather, with Scotland and Northern Ireland likely to see spells of heavy rain, while England and Wales will remain largely dry, sunny, and increasingly hot.
Temperatures could reach 30C (86F) on Saturday, before climbing into the low 30s in southern England on Sunday.
Highs of up to 30C are possible across parts of England and Wales on Tuesday, increasing to 32C to 34C (90F to 93F) on Wednesday, with the highest temperatures currently expected on Thursday.
'Several more areas' could move into drought
The forecast comes as the Environment Agency confirmed that last week was the seventh consecutive week in which England received less than 10mm of rainfall.
The agency suggested "several areas" could move towards or into drought next week, in addition to the half of England already in drought.
It is already planning to introduce formal restrictions on the amount of water that can be removed from natural sources in parts of East Anglia to protect exceptionally low river flows.
Reservoir storage in England has also dropped to 69%, down from 72% a week ago and nearly 12% below the long-term average for this time of year, the agency said on Friday.
Storage stood at 85% at the end of June, which was the average for that point in the year, but has fallen steadily in the past few weeks.
Five reservoirs are now classed as having "exceptionally low" levels: Blagdon in Somerset (54% full); Clywedog in Powys, which provides water for central and North West England (62%); the Dove group of reservoirs in Derbyshire (74%); Hanningfield in Essex (60%); and Wimbleball in Somerset (50%).
Wildfires to become 'exceptionally severe'
Last month was the driest July for England and Wales since records began in 1836, with 19 counties across England recording effectively no rainfall - 1mm or less, according to provisional figures released by the Met Office.
It comes after several wildfires have broken out in the UK this summer, with large blazes in Suffolk, on the Isle of Wight, in the Rhondda Valleys, South Wales, and the Cairngorms National Park in Scotland.
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The Fire Severity Index, which evaluates how severe a wildfire could become if one were to ignite under current weather conditions, says fires on Sunday are likely to become exceptionally severe - its highest ranking - for almost all of the East and South East of England.
The Met Office data includes rainfall, temperature, and humidity, but the index does not cover the chances of wildfires occurring.
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