Heavy thunderstorms could cause danger to life, sudden flooding and power cuts as forecasters upgraded a weather warning.
The Met Office issued an amber alert for parts of England with intense storms bringing the British heatwave to an end.
Thursday saw the highest temperature recorded in the UK since 2015, with the mercury in Faversham hitting 35.3C (95.5F).
The hot weather led to an amber warning for thunderstorms being issued for east England and North Yorkshire until 9pm on Friday.
Yellow warnings are in place for most of England into the early hours of Saturday, and in north-east Scotland until 2pm.
An amber severe weather warning for #thunderstorms has been updated: https://t.co/QwDLMfRBfs. Stay #weatheraware @metofficeuk pic.twitter.com/YdMOJ5OwEH
— Met Office (@metoffice) July 27, 2018
Torrential downpours were forecast, with as much as 30mm of rainfall in one hour and 60mm in three hours, while large hail, frequent lightning and strong, gusty winds will be additional hazards.
Forecasters said that those in the areas covered by the warnings can expect flooding of homes and businesses, which could happen quickly.
— Met Office (@metoffice) July 27, 2018
Delays or cancellations on transport, difficult driving conditions and power cuts are also possibilities.
Yellow warnings for rain have also been issued for Northern Ireland on Saturday, with thunder possible.
Singer Sir Tom Jones was forced to cancel a concert due to the wet weather.
York Racecourse tweeted that the show had been called off because of the “thunder and lightning storms”.
Will Lang, chief meteorologist at the Met Office, said conditions would change over the weekend.
He said: “While Saturday will be a pleasant, but breezy day in many places, we are in for a change on Sunday with a wet and windy day in stark contrast to what has been typical recently.
“We have already issued a Met Office weather warning for the impacts of wind and rain in south-west England and south Wales. Southerly winds will gust at around 35-40 mph and up to 50mph for exposed coasts and hills. Rain will be most persistent over hills and parts of the Brecon Beacons and Dartmoor could see as much as 60-80mm of rain.”
Tony Wafer, RNLI community safety manager, said: “If you are heading to the coast this weekend, please find out where your nearest lifeguarded beach is and ask the lifeguards for advice on conditions, especially if you are going in the water.
“RNLI lifeboats and lifeguards have had four times as many rescues of inflatables so far this summer than they did last year.”
