The M23 motorway is shut in both directions close to Gatwick Airport due to flooding on one of the busiest days of the year for drivers.
A Met Office weather warning for rain is in place and nearly 240 Environment Agency flood warnings are in place.
It comes on so-called "Frantic Friday" when millions of Brits head home at the start of the weekend before Christmas.
Up to 1.2in (30mm) of rain is forecast to fall in southern England this morning, and further alerts are in place over the weekend.
There is also flooding on train tracks at East Grinstead in West Sussex affecting Southern Rail services.
Three yellow warnings for rain are in place in the UK today, one covering London, the South East and East Anglia, one in Cornwall, and one in the east of Scotland.
The Met Office warns that homes and business could be flooded, and some buildings could be damaged.
Trains and bus services face further delays and cancellations.
Some isolated communities could even be cut off by flooded roads while driving conditions will be difficult due to spray.
A Met Office spokesman said: "There will be some showers around but nowhere near as much rain as today. More rain is coming from the south through the night and tomorrow.
"Most of the rain will be in the eastern part, eastern England and Scotland, gradually improving throughout the day.
"It does look, as we head into next week, that the weather will turn drier."
Another yellow warning is in place on Saturday and Sunday for the south of England.
Highways England has warned drivers the M23 could remain closed for "several more hours" due to substantial flooding.
A maintenance team is currently trying to clear the water from the submerged motorway.
In a statement on Twitter, it said: "M23 remains closed both ways between J10 and J11 whilst we continue work on clearing the floodwater.
"We've several pumps working on removing the water and the stream of water on to the road has been stemmed.
"No current expected reopening time at present but we're making progress."