Parts of the Midlands and South West could reach 33C (91.4F) between Tuesday and Thursday, which is hotter than top European holiday destinations such as Marbella, Mykonos and Tenerife.
The mercury will remain above 31C (87.8F) during the week, with forecasters suggesting the next few days could be the hottest of the year.
And even though the Met Office warning does not specifically cover Lancashire or the North West, the heatwave is likely to hit the mid-to-high 20s between Tuesday (July 20) and Thursday (July 22) this week.
In central and the east of the county, including Preston and Burnley, temperatures are expected to reach 26C, with northern and coastal parts such as Blackpool and Lancaster feeling slightly cooler with highs of 23-24C.
Conditions will begin to cool down into the weekend, as temperatures drop slightly to 21C in most parts of Lancashire, as light cloud and showers move in.
The Extreme Heat Warning coincides with an extension to Public Health England’s Heat Health Alert through to Thursday, reflecting the risks to the health sector in England.
Dr Owen Landeg, Scientific and Technical Lead at PHE, said: “Everybody can be affected by high temperatures and most people are aware of good health advice for coping with hot weather. However, it’s important to keep checking on those who are most vulnerable such as older people and those with heart or lung conditions.
“As we experience the first hot weather episode of the year, it’s important for everyone to remember to adapt their behaviours. This is particularly important during the pandemic with many people self-isolating.
“Most of us want to enjoy the sun. Remember to look out for signs of heat exhaustion and follow our simple health advice to beat the heat.”