Temperatures are likely to soar to 33C when a "Spanish plume" hits the UK on Friday, forecasters said.
The Met Office says it will be the first hot spell of weather of the year for some parts of the UK as high pressure dominates.
A hot plume of air is heading up from Spain and Portugal where it is forecast to be absolutely roasting.
From temperatures in the low 20s Celsius on Tuesday the temperature is forecast to pick up day on day.
The Midlands could see temperatures in the mid 20s on Wednesday, 26 on Thursday and 27-29 on Friday.
Hottest temperatures are forecast for the south and east of the UK where the temperatures are set to rise into the low 30s Celsius.
Levels of skin burning ultra violet light are also forecast to be high or very high.
Forecasters say there is uncertainty about what the weather will do at the weekend.
Met Office Deputy Chief Meteorologist Dan Rudman said: “Temperatures will rise through the week, becoming well above-average by Friday when many parts of the southern half of the UK are likely to reach 30C or even 33C in isolated spots.
“This is the first spell of hot weather this year and it is still unusual for temperature to exceed these values in June."