“Severe” thunderstorms and localised flooding may follow a heatwave across parts of the country, the Met Office has warned.
The weather service said thunderstorms on Thursday could feature “frequent lightning, torrential downpours and large hail”, as it issued weather warnings.
Temperatures on Wednesday are expected to equal the hottest day of the year so far, as a heatwave spreads across the UK.
Daytime temperatures are forecast to reach highs of 30-32C in parts of central and southern England and eastern Wales.
The Met Office said “many more locations” would meet its heatwave criteria following Wednesday’s warm weather.
A yellow thunderstorm warning is in place for London, Essex and much of the South East on Wednesday from midday until 11.59pm.
Met Office deputy chief forecaster Dan Holley said the most intense impacts on Thursday were “likely to be focused on central, southern and south east areas of England”.
He added: “Here some locations will witness torrential downpours, large hail and frequent lightning.
“A few places could see 50-100mm of rainfall in a few hours.”
In a post on social media, the Met Office said the thunderstorms “may be severe on Thursday with frequent lightning, torrential downpours and large hail giving localised flooding”.
It added: “But the nature of these ‘hit and miss’ thunderstorms means that some of us will stay dry.”
Tuesday was the warmest day of the year so far with a temperature of 32C recorded at Kew Gardens and Heathrow.
Met Office spokesman Oli Claydon said the hottest temperatures for Wednesday were likely to “shift further west” to regions such as the West Midlands and parts of the West Country.
Mr Claydon told the PA news agency: “We’re expecting to see many more locations with their temperatures today taking them over the heatwave criteria – with three days over their threshold temperature.
“We’re expecting it to be fairly widespread across parts of the south of England, particularly the South East.”
Mr Claydon said it was a “pretty uncomfortable night for sleeping” in the South with “high temperatures, but also high humidity”.
He added: “It was very nearly what we call a tropical night where temperatures don’t get below 20C.”
The Met Office spokesman said a weather front approaching from the northwest on Friday would bring “fresher air and the end of the heat” – with lower temperatures expected by the weekend.
The London Fire Brigade urged people to be careful while swimming as the hot weather continues.
Mr Carter added: “Often, young people are encouraged by their peers and aren’t aware of the dangers – no matter how experienced a swimmer you are, it can be impossible to know how deep water is or how fast it is moving.
“Even on hot days, the water temperature can be cold enough to bring on cold water shock, causing panic, anxiety, disorientation and loss of muscular control. This can cause you to gasp for air and inhale water.”
Charities have also issued warnings as the heatwave spreads, with Shelter advising it could pose a “big risk for people who are homeless on the streets”.
Dogs Trust also warned there was “no safe amount of time to leave dogs alone in a car” during the hot weather.
