A severe weather warning has been issued for thunderstorms on what is set to be the hottest day of the year.

The Met Office put out the alert, covering a large swathe of Britain for Friday and Saturday, as forecasters predicted a blast of very humid air could bring highs of 27C.

It said there is a chance of “significant disruption” from surface water flooding, while lightning and hail could also cause problems across southern England, south Wales, the Midlands and the east of England.

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.

There is potential for high amounts of rainfall in very short periods in some areas, such as 20 to 30mm within an hour, added the Met Office.

The conditions are set to be caused by an area of warm, humid air moving across France into southern Britain overnight on Thursday.

Met Office forecaster Dean Hall said: “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.”