Parts of the UK have experienced a third of a month's rainfall in just 12 hours overnight.
London and the surrounding areas were worst affected by the downpour, while Wales and north-west England were also hit.
The Met Office issued severe weather warnings for heavy rain in London, south-east England, south-west England and Wales.
Parts of the M20 motorway near Wrotham, Kent have been closed since the early hours. The westbound carriageway from junctions 2-1 was shut at 3.20am because of the wet conditions. The westbound A27 at Polegate near Eastbourne, East Sussex, was closed just after 7.30am but has now been reopened.
Three Environment Agency flood warnings remain in place for East Sussex. They are for the River Ouse at Barcombe, Lewes, the Cuckmere River and Bull River at Hellingly, Hailsham, and the River Uck and confluence with the River Ouse at Isfield, Uckfield.
The wet weather is a consequence of settled low pressure systems moving from west to east across the southern part of the UK.
Daniel Adamson, a forecaster with MeteoGroup, the weather division of the Press Association, said: "In the 12 hours up to 6am, central London had about 10mm of rain. Places just to the south and east of London had up to 20mm. That is probably getting on for a third of a month's rainfall.
"The rest of the country had a lot less but there has been some rain in Wales and across the north-west of England, and some of that has fallen as snow. But the southern counties have had the most of it."
More rain is expected to arrive on Saturday night and stay into Sunday morning.
Experts said that although the weather is wetter than normal, temperatures are about average for this time a year at around 6C (42.8F).
