The Met Office has issued a yellow warning of rain for London and the Southeast of England as torrential rain is set to hit the region from Sunday evening.
Forecasters say: "Organised bands of heavy showers and the odd thunderstorm, look like continuing to feed along the English Channel and then coming increasingly into parts of southern England as the flow backs more southerly overnight and at first on Monday."
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Up to 10mm of rain could fall within an hour and perhaps 20 to 30 mm through the period in a few spots.
The rain looks set to continue for the next few days before an icy blast hits Britain.
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Mel Harrowsmith, at the Met Office, told the Independent: "For many the weather during the coming week is likely to be the first experience of winter, with either frost, wintry showers or even snow affecting many areas of the UK."
Meanwhile, meteorologist Leon Brown, at The Weather Channel, says: "Some severe frosts are likely over the north with -10C possible. We will still have to watch the threat of a more widespread spell of sleet and snow moving from the west across south and central areas late next week and weekend.