Many parts of England and Wales could be hit by storms and flash flooding over the next 24 hours, the Met Office has warned.
The Environment Agency has issued 54 flood alerts, the majority of which are in the South East, South West and Midlands.
Met Office chief meteorologist Paul Gundersen told the BBC: "We're expecting heavy thundery showers through the weekend with the potential for torrential downpours, large hail, frequent lightning and locally strong gusts.
"Not everywhere will see thunderstorms but where they do form we could see significant rainfall in a short space of time, which could cause localised flooding."
The Daily Mail reports that the Environment Agency has warned 30mm of rain could fall in an hour in some areas.
Friday was confirmed as the hottest day of the year so far, with temperatures reaching 32C in Gravesend, Kent.
Weather forecasters have predicted heavy thunderstorms in eastern areas on Saturday. Temperatures are expected to be lower, but "it will still be very humid".
Meanwhile, Heathrow Airport has said that it will cancel one in five flights during a two-hour period on Saturday due to thundery weather.
"We have therefore requested that airlines reduce their flight schedule by 20% between 4pm and 6pm. Passengers due to travel on July 19 are urged to check the status of their flight with their airline before travelling to the airport."
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