Friday could be Britain's warmest ever July day - and possibly even its hottest day on record.
While temperatures of 35C to 36C are widely forecast for the south east, "there is a possibility that the July maximum temperature - 36.7C - will be reached," Bonnie Diamond, a press officer at the Met Office, said.
"There is also a 20 per cent possibility the all-time high could be reached."
The hottest temperature on record is 38.5C in August 2003.
But torrential downpours are forecast to hit parts of the UK, with as much as 30mm expected to fall in just an hour.
"Scattered thunderstorms will develop and there is a danger of flash flooding," Ms Diamond said.
A yellow thunderstorm warning is in place for eastern and northern parts of England from Friday afternoon, which cautions that rain could cause widespread travel disruption and lightning could cause power cuts.
With a chance of hail and strong winds alongside the torrential downpours, the temperature could drop back to the high 20s, forecasters said.
"The weekend is not going to be as hot as the end of the week, since there is a cold front pushing east across the UK bringing fresher air," Ms Diamond said.
However, Saturday and Sunday were "looking largely dry", with only the odd isolated shower, she said.
The heatwave scorching Britain comes at the same time as extreme hot weather grips northern Europe.
An amber "heat health watch" warning remains in place for parts of England, with people being warned to try to stay out of the sun between 11am and 3pm.
Nurses have warned that some hospital wards have reached more than 30C, leading to patients and relatives passing out or vomiting.
The Local Government Association said social workers, community wardens and maintenance staff are all on high alert to identify those who could be struggling in the heat.
And pet owners have been urged to avoid exercising their dogs and other animals in the hottest parts of the day due to the risk of burnt paws on scorching pavements, as well as the chance of heatstroke.
UK homes and cities need to be prepared for a future of more heatwaves as temperatures rise, the government's climate advisers have warned.
The Committee on Climate Change has repeated its call for new standards on heat risks for new homes, while action is also needed to keep existing houses cool and create more green space in cities to prevent urban overheating, it said.
And as the harvest begins early in many places following the driest first half of summer on record, farming leaders warned crops are being hit.
They added that reservoirs for watering vegetables were running dry and livestock owners were having to use winter feed for their animals as summer grass had withered away.
Weather extremes including record temperatures, heatwaves and drought are being seen across the northern hemisphere in the first half of summer, hitting health and agriculture and causing dangerous wildfires.
Experts have warned climate change was "loading the dice towards extreme weather," with rising global temperatures fuelling heatwaves.
In Ireland, heatwaves were recorded at 15 weather stations, with five or more consecutive days with temperatures above 25C, and an absolute drought at all its stations, the World Meteorological Organisation (WMO) said.
The UK has seen the driest half of summer on record, with just 47mm (1.85 inches) of rain between 1 June and 16 July.
The Met Office said several places have had 54 consecutive dry days, starting on 30 May, including a few which have had less than 1mm of rain in the entire 54-day period - the longest spell since 1969, when 70 days passed with no significant rainfall.
The longest run of days with no rain at all this summer so far is 48 days at Brooms Barn, near Bury St Edmunds, since 5 June.
The prolonged warm, dry weather is caused by an area of high pressure sitting to the west of Britain, leaving the country basking under cloudless skies.
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