Temperatures could hit 40°C, meaning a danger to life is likely, with the risk of illness not limited to vulnerable people.
The warning covers London and the Midlands and goes as far north as Manchester and York.
The Met Office has issued an amber weather warning for Sussex on Monday and Tuesday.
According to the Met Office’s weather warnings guide, an amber warning means there is an increased likelihood of impacts from severe weather, which could potentially disrupt your plans.
This means there is the possibility of travel delays, road and rail closures, power cuts and the potential risk to life and property.
According to the latest forecast, the extreme weather in Sussex should begin at the beginning of next week, with temperatures forecast to reach 35°C on Monday and Tuesday respectively.
Extreme weather warnings are in place for the whole of England and Wales from Monday at 12am to Tuesday at 11.59pm.
But these scorching temperatures won’t last until the end of the week, it has been confirmed.
BBC Weather has forecast that temperatures will feel far cooler, with thundery showers expected in Sussex by Wednesday. The rainfall is expected to abate on Thursday before continuing into the weekend.
Temperatures are expected to peak at 25°C on Wednesday before dropping to 22°C on Saturday.
Speaking to the Express, Jim Dale, of British Weather Services, said: “Wednesday looks to be the day, it will be coming in from France but it’s still a relatively long way off.
“It should arrive in the evening after a hot or sultry day.”
The Met Office’s long-range forecast between July 16 and 25 has also predicted thundery showers.
The forecast says: “Thundery showers are occasionally possible in the south and southwest too, spreading erratically northwards and eastwards.
“It is uncertain how long the very hot weather will last, but it is likely that much of the UK will see a return to cooler and more widely unsettled conditions during the week”