Torrential downpours could drench London on Saturday (September 3), forecasters have warned.

A yellow weather warning has been issued for the capital and much of southern England, with up to 40mm of rain predicted within three hours in some places.

The weather warning covers the period from 3pm to 9pm, though with the downpours spreading from the west, London is more likely to be affected towards the end of that period.

The Met Office said heavy rain affecting parts of Devon and Cornwall would move east through the rest of the afternoon, with further outbreaks possible during the evening.

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It warned about the risk of flash floods, which it said could cause transport disruptions.

Although forecasters said not everywhere in the south would be inundated with the same amount of rain, they claimed a few places could get 15-20mm within an hour and as much as 30-40mm over three hours.

The rainfall could mean there is a damp afternoon in store for revellers at the ZEE London Mela, which is taking place in Wembley Park this year rather than its usual home of Gunnersbury Park.