The heavy downpour is causing vehicles to drive slowly on the M25, causing disruption on the motorway. It comes after the Met Office put out two weather yellow weather warnings for flash floods.
Footage shows the central part of the motorway underwater, as vehicles make their way through the water.
The torrential rain comes after days of heatwave across the country.
Weather forecasters warned Britons to brace for “explosive thunderstorms and heavy downpours” today.
A weather map from the Met Office shows flashfloods are hitting swathes of England, including the south coast.
READ MORE: Victoria station UNDERWATER: Travel chaos as flash floods engulfs city
The amber thunderstorm warning issued by the Met Office covers most of the south east of England, including parts of London.
The warning is in place from 11am until 10pm on Wednesday.
30mm to 50mm of rain could fall in just an hour, while a few places may see more than 100mm in a few hours.
Met Office spokesman Grahame Madge said: “Heavy showers and thunderstorms will lead to some localised flooding and general disruption.
“Not everywhere will experience heavy downpours, or even see rain, but those affected areas could see 30mm to 50mm of rain falling in an hour.”
Twitter user Andrew @AndrewsVisual took to Twitter, saying: “The Flash flood that just happened in London is insane… the rain was incredibly mad.”
Islington Council tweeted: “Heavy rain is causing large pools of water in several locations in Islington and Finsbury Park – please take care on your journey home.”
The torrential rain comes after a drought was officially declared across most of England following the driest month of July in 50 years and the driest first half of the year since 1976.