The UK's Met Office said there was an "80 per cent probability" of heatwave conditions this week in parts of England.
The forecast triggered a "level 2" alert, with the Met Office warning: "Heatwaves can be dangerous, especially for the very young or very old, or those with chronic disease."
A spokesperson said: "Temperatures are expected to build on Tuesday and into Wednesday, with the hottest day of the year so far expected on successive days this working week."
The heatwave is caused by a warm front and tropical continental air mass from Europe pushing across the country - bringing with it high temperatures and humidity.
Coastal areas will be cooler, but Northern Ireland and Scotland will still experience temperatures in the mid to high 20s (degree Celsius).