A sizzling 36C (96.8F) is forecast for the south east, and a thunderstorm warning is in place for eastern and northern parts of England from Friday afternoon.
The Met Office is advising people to stay out of the sun and keep drinking fluids.
As temperatures soar above 30C for the third consecutive day, the Met Office has extended the heatwave alert for another 24-hours into the weekend.
While the United Kingdom is sometimes mocked for being bad at dealing with hot weather, these heat health warnings are far from UK-exclusive.
This hot weather is then expected to trigger thunderstorms throughout various parts of the United Kingdom towards the end of this week.
"Temperatures are now expected to peak across eastern parts on Friday, with the chance of some exceptionally hot weather developing".
Public Health England urged Britons to do their best to stay out of the sun until the heatwave is over - especially between 11am and 3pm.
Instead, people have been advised to open their windows when it is cooler at night.
Portsmouth residents warned to ‘stay out of sun’ as Met Office issues heatwave alert
If there is anyone you know who may be at risk, for example an older person living on their own, make sure they know what to do.
The Met Office says there is a 90 per cent probability of heatwave conditions between 9am on Monday and 9am on Friday in parts of England.
"Whilst many places will remain dry and hot, the thunderstorms on Friday could lead to torrential downpours in places with as much as 30mm of rainfall in an hour and 60mm in three hours".
"Large hail and strong, gusty winds will be additional hazards".
A Met Office spokesperson said: "There is a small chance that homes and businesses could be flooded quickly, with damage to some buildings from floodwater, lightning strikes, hail, or strong winds". But it is set to get even hotter, with temperatures expected to peak tomorrow and Friday thanks to a plume of air dubbed the "Mediterranean Melt".
Having been a 1/2 shot yesterday, it's now just 1/3 Britain will experience 35C or hotter before the week is out.
The UK is bracing itself for another day of scorching temperatures as a heatwave continues to hit the country.