A yellow warning is in place for «potentially intense» thunderstorms in large parts of the country – and some places could have up to 40mm of rainfall in less than two hours.
The Met Office said there could also be «frequent lightning, large hail and strong winds».
The alert lasts until 3am on Sunday and covers parts of North East and North West England, the Midlands, Yorkshire, northeast Wales, and the Scottish Borders.
The Met Office urged those in the warning areas to consider if their location is at risk of flash flooding and to prepare accordingly.
Forecasters have said the heatwave in parts of England and Wales provides «perfect conditions» for thunderstorms.
On Saturday, the UK had the hottest day of 2025 so far, with a temperature of 33.2C (91.7F) recorded in Charlwood, Surrey.
Scores of rail passengers were evacuated after some services were halted following a fault on a train.
And thousands of people watched the sunrise over Stonehenge in Wiltshire to celebrate the summer solstice, marking the year’s longest day.
Met Office meteorologist Jonathan Vautrey said «hot and humid air is being dragged up» from parts of continental Europe.
«But we also have low pressure out in the Atlantic, and that is driving weather fronts across the UK, providing instability in the air and the perfect conditions to start sparking off some thunderstorms as that hot and humid air rises rapidly,» he added.
People enjoy the warm weather on a beach in Margate, Kent. Pic: PA
The east of England, including London and the South East, could experience a «tropical night», although most of the country will see «a lot cooler» and «a lot fresher» conditions, Met Office forecaster Dan Stroud said.
«In one or two spots, that could potentially be a tropical night, which is where the overnight minimums actually fail to drop below 20 degrees (centigrade),» he added.
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Sunday will see a day of «sunny spells and showers» with highs of around 27C and 28C, Mr Stroud said.
An amber heat-health alert for all regions in England remains in place over the weekend.
The alert, issued by the UK Health Security Agency (UKHSA), has warned that significant impacts are likely during the alert period across health and social care services, including a rise in demand.