The Met Office put out the yellow alert - meaning be aware - for southern England, south Wales, the Midlands and the east of England - but the North may get a soaking too.
There is a chance of “significant disruption” from surface water flooding, while lightning and hail could also cause problems, the weather warning noted.
Isolated severe thunderstorms are likely to begin from noon on Friday, followed by a band of more widespread heavy rain until the early hours of Saturday morning.
The North may well get showers - some of them heavy - on Saturday and Sunday, say forecasters.
There is potential for high amounts of rainfall in very short periods in some areas, such as 20 to 30mm within an hour.
The conditions are set to be caused by an area of warm, humid air moving across France into southern Britain overnight on Thursday.
The mercury could soar as high as 28C, beating the year’s previous highest point of 26.7C recorded in Frittenden, Kent last Friday.
Met Office forecaster Dean Hall said: “Friday will start very warm and humid in the South but end up with thundery rain .
“Maximum temperatures, particularly in the South East, could reach 25C or 26C. There might be somewhere reaching 27C or even 28C.
“South Wales could see 24C, with 21C in the Midlands and the North.”
He added: “But the thing that may spoil it is thunderstorms triggered by the heat and humidity. The atmosphere will become unstable.
“It looks like a more persistent band of heavy downpours as we go through Friday evening into Saturday.”
Bookmakers are confident that Friday will be the hottest day of the year, with Coral offering odds of 8-11.
Punters can also get odds of 7-4 that the temperature reaches at least 28C.
Coral’s Nicola McGeady said: “The great British weather has disappointed us so far but the odds suggest that will change by Friday, with the hottest day of the year so far expected.
“Whether it lasts or not remains to be seen.”