The amber weather alert for thunderstorms covers Cornwall, Devon and Somerset. It is in place from 2pm on until 8pm on Monday.
The Met Office warned that “thunderstorms and heavy showers are likely to cause travel disruption and, in a few places flooding”.
The forecaster warned that homes and businesses should be prepared for flooding, with damage expected to some buildings in the region.
It added: “Fast flowing or deep floodwater is likely, causing danger to life.”
The Met Office then warned that “disruption” to travel could become widespread.
It said: “Where flooding or lightning strikes occur, delays and some cancellations to train and bus services are likely.
“Spray and sudden flooding probably leading to difficult driving conditions and some road closures.”
This comes after the Met Office issued a series of yellow weather alerts across the UK.
The weather service initially issued a yellow warning for thunderstorms for south west England on Monday morning.
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The yellow alert then became a rain warning towards the English border with Scotland.
The yellow warning was issued for the Borders, Lothian, Dumfries and Galloway, as well as Fife and Strathclyde.
Despite the downpours, temperatures are expected to remain high throughout the middle of the week.
The cooler temperatures will be seen across the Scottish Highlands, with numbers hovering in the mid-teens.
London will be among the warmest, just touching 25C despite the thunderstorm warnings.
Much of the country will see temperatures in the low twenties, and often high teens.