"Severe thunderstorms" are expected to sweep across the country on Friday and the Met Office has issued a weather warning for rain.
Forecasters have issued the yellow warning for rain, from midday on Friday (June 12) to 7am on Saturday (June 13), as temperatures are predicted to soar to around 27C in parts of the country.
The warning said: "Isolated severe thunderstorms seem likely to develop on Friday, these succeeded by a band of more widespread heavy rain and occasional thunderstorms, moving gradually north later in the day, affecting much of the highlighted region [most of the South] into the early hours of Saturday morning.
"The public should be aware that there is a chance of some very localised significant disruption, mainly from surface water flooding. Lightning and hail could also cause some impacts."
It comes as the Met Office forecasts temperatures of up to 27C on Friday - with Britain likely to be hotter than Portugal - as a mini heatwave hits.
But the hot weather is also expected to cause thunderstorms and heavy rain.
The Met Office's chief forecaster said: "An area of warm, very humid air is expected to move northwards across France into southern Britain later on Thursday and into Friday.
"Whilst isolated thunderstorms are possible from Thursday, these become more likely and potentially more severe by Friday afternoon, with the potential for 20-30mm falling within an hour or so, probably on a very localised basis.
"Indications are that a more organised band of heavy rain and thunderstorms will follow by Friday evening and overnight into Saturday, bringing 25-50mm of rain more widely, again with the potential for some very high short-period totals.
"Given the complexity of the developments leading up to this event, ideas on areas at greatest risk of heavy rainfall and severe thunderstorms may well change, and this warning will be updated accordingly."
See our five-day forecast here.