The Met Office say rain, lightning, and flooding could cause travel disruption amid the hot weather.
Yellow weather warnings for thunderstorms have been issued for much of England and Wales amid a heatwave gripping large parts of the UK.
Heavy showers, lightning, and thunder are forecast on Wednesday night and into Thursday, with travel disruption and power cuts possible.
Flood alerts are also in place in parts of the Midlands.
The warm temperatures of recent days are set to continue into Friday, with peaks of 28C expected in southern England.
The yellow warnings affect central, northern, southern, and south-eastern England, as well as central and eastern Wales.
The Met Office said some areas could see difficult driving conditions and road closures, as well as delays and cancellations to train and bus services.
It also warned there could be damage to buildings from fast-flowing water, lightning strikes, hail, or strong winds, and that the flooding could pose a danger to life.
Homes and business could see power cuts and losses of other services, the organisation added.
Dan Holley, a deputy chief forecaster at the Met Office, said the thunderstorm warning had been issued for a broad area but that not all of it would be affected.
“The most intense impacts are likely to be focussed on central, southern and southeast areas of England,” he said.
“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.”
A separate yellow warning that is already in force in east and south-east England is also due to remain in place until 23:59 BST on Wednesday.
The Environment Agency has also issued eight flood alerts for parts of Nottinghamshire, Derbyshire, and Loughborough.
The rivers affected include the Trent, Maun, and Leen, and the agency said the areas most at risk were low-lying agricultural land, roads, and footpaths close to small rivers and streams.
People in the area are advised to avoid using low-lying footpaths near local watercourses and to plan driving routes to avoid low-lying roads near rivers.
The UK recorded its hottest day of the year on Tuesday, when measuring stations in both Heathrow and Kew Gardens, south-west London, reached 32C (90F).
The Met Office defines a heatwave in the UK as “when a location records a period of at least three consecutive days with daily maximum temperatures meeting or exceeding the heatwave temperature threshold.”
For London and the surrounding region, that threshold is 28C – which has been surpassed since Monday. Temperatures had been expected to remain above that level through to Friday.
The hot weather is expected to cool slightly into the weekend, with highs of around 23C forecast for Saturday.
The Met Office said there would still be a risk of isolated thunderstorms in the east and south-east of England, but that their frequency and impact would be much reduced.
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