It is possible that half a month’s rain will fall within an hour in parts, with the extension of weather warnings

The Met Office said half a month’s rain could fall within an hour in some parts of the UK as thunderstorms move across southern England.

The forecaster said there was a “small chance” that lives would be at risk.

A yellow thunderstorm warning is in effect across southwest England and south Wales until 6pm on Sunday.

A similar warning has been issued for London, the south-east, eastern England and the East Midlands until 6am on Monday.

Homes and businesses may be at risk of flooding quickly during heavy rain, although the risk is said to be low.

In some parts of southern England, 30 to 40 mm of rain is expected to fall, which is equivalent to at least half the September average of 55 to 60 mm.

Buildings can also be damaged by lightning, hail, or strong winds.

There is also a “small chance” of power outages or loss of other services to homes and businesses, while some communities may be cut off by floodwaters, the forecaster said.

People planning to travel face the possibility of sudden delays or cancellations of trains and buses.

Roads may be closed at short notice due to drizzle and flash flooding and “difficult driving conditions” are expected on roads that remain open.

Heavy rain dumped heavy rain on southwest England on Sunday morning, with localized flooding in south Devon.

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The rain band is expected to move into south-east England on Sunday afternoon.

Jonathan Vautrey, a meteorologist at the Met Office, said: “There is a potential for these thunderstorms to become severe” and bring “gusty winds with very heavy rainfall”.

They will move relatively quickly, making it difficult to pinpoint exactly where the hardest-hit places are, Vautrey said.

“It’s definitely worth keeping up with expectations,” he added.

“Although the warning area covers the whole of south-east England, not everywhere in that area may see the most severe thunderstorms.

“It’s a good idea to check these things right away before setting off on your trip so you’re aware of where the most severe thunderstorms could occur.

“Make sure to be careful because the weather can change in very short periods of time, and be prepared for those gusty winds and possible large hail storms.”

Conditions are expected to remain “blissy at times” early next week but are likely to be more active.

More storms are likely as the remnants of Hurricane Lee, which struck New England in the US and eastern Canada, is set to move across the UK between Tuesday and Thursday.

It will not become a hurricane by the time it reaches UK shores.

“That will be picked up by the jet stream,” Mr. Vautrey said. Rain may be heavy in places with a risk of further thunderstorms.

“It could be quite an unsettled fall week.”