On Wednesday, Weatherquest's Adam Drury said: "On Friday there is an initial low risk of a few spots of rain to the west and thunder storms in the afternoon with good chance of heavy storms across the county".
The UK could experience its hottest day ever on Friday and is likely to set a new record for the highest July temperatures, the Met Office has announced.
The warning will remain in place between 2pm today and 2am on Saturday morning.
Parts of Britain are forecast to bask in temperatures of 35C (95F) later this week, surpassing Monday, which was the hottest day of the year so far.
The British record is 38.5C set in August 2003 near Faversham in Kent, southeast England, while the highest July temperature was 36.7C in 2015 at Heathrow airport.
"Further thunderstorms are possible until late evening, however, and this risk is addressed in the yellow warning already in force".
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"Where the thunderstorms occur, torrential downpours are possible with 20 mm of rainfall in 1 hour and perhaps 30-40 mm in 2 or 3 hours". Heavy rain and thunderstorms in the small hours of tomorrow will continue to bring the temperature down, making for a slightly cooler and rainy weekend in the capital, with some sunny intervals.
Large hail and strong, gusty winds are additional hazards.
A high pollution warning was issued for London on Thursday, due to a mix of toxic air, extremely high temperatures, emissions from the continent and a lack of cloud.
The Met Office says temperatures could peak until the very end of the week.
Met Office is warning that there is a 90 per cent probability of heatwave conditions across Portsmouth and Hampshire over the coming days.
Meanwhile, London's fire chief has called for barbecue bans to be imposed in the capital's parks after a record-breaking number of grass fires.