The Met Office has extended a yellow weather forecast for thunderstorms – with major parts of England and Wales affected.
Forecasters say heavy rains can cause travel disruptions, and flooding is also possible.
The weather forecast area has been extended to the west and north, and is valid until 23:59 tonight.
Where thunderstorms occur, up to 40 mm of rain can fall in as little as an hour – rising to 60 mm over a period of two or three hours.
“These torrential rains may be accompanied by frequent lightning, while gusts of wind and hail may also affect a few places,” the Met Office warning added.
The thunderstorms are expected to occur mainly in the afternoon and early evening, with forecasters warning that their exact location “will be difficult to pinpoint”.
Changes to the warning area are possible in the coming hours.
Scammers at Glastonbury have been told they may have to seek refuge as thunderstorms wash over the country.
Festival goers were asked to pack for all eventualities – light showers are predicted Friday and Saturday, with heavier rain on Sunday.