The Met Office has warned there could be power cuts and flooding as a 12-hour thunderstorm warning for London came into force at midday on Tuesday.
The yellow alert covers a huge swathe of south-east England, including the capital, and runs through to midnight.
The predicted heavy rain comes after the capital basked in 21C sunshine on Monday following a sunny weekend.
London also enjoyed scorching temperatures earlier this month, as the mercury reached 26C for the first time in 2024.
The Met Office has warned the heavy rain and thunderstorms on Tuesday could lead to disruption.
It says sudden flooding could result in road closures and delays and cancellations to train and bus services.
It adds: “There is a slight chance that power cuts could occur and other services to some homes and businesses could be lost
“There is a small chance that homes and businesses could be flooded quickly, with damage to some buildings from floodwater, lightning strikes, hail or strong winds.”
The forecaster has warned Tuesday will be a “messy day of weather” with thunderstorms rumbling across the UK.
Lightning strikes could cause disruption in the worth hit parts of the country, according to Met Office meteorologist Alex Deakin.
Thunderstorms and intense downpours are also set to hit parts of south-west England and Northern Ireland.Up to two inches of rain could fall over a few hours in Cornwall, Devon, Dorset, Plymouth, Somerset, Torbay and in western parts of Northern Ireland on Tuesday.
Frequent lightning strikes and hail may also be seen, the Met Office’s yellow thunderstorm warnings said.
Met Office forecaster Amy Bokotan said: "The downpours could be slow moving, and that's the main trouble - the wind's quite light at the moment so when they do form, they could be quite slow moving and intense."
She added: "Where you see them they could be quite intense and dramatic, but not everywhere's going to see them and some places just down the road could be completely fine and shower and thunderstorm-free."
