Heavy rain could cause travel chaos throughout the weekend as it coincides with a strike by train drivers on Saturday.
The Met Office has issued yellow weather warnings for southern England and southern Wales and Saturday and Sunday. Parts of Scotland are under a yellow weather warning on Saturday.
It said downpours would likely cause flooding on roads and make journey times longer, with flooding of homes and businesses also possible.
Four flood warnings and 56 flood alerts have been issued in England.
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The weather warnings on Saturday coincide with train drivers from 11 companies belonging to the Aslef union striking in a long-running dispute over pay.
Passengers have been urged to plan ahead and check before they travel throughout the entire weekend.
Trains that are running will start later on Saturday morning and finish much earlier than usual, with services typically running between 7.30am and 6.30pm on the day of the strike.
Rain, which is expected to be heavy at times, is set to hit parts of south Wales and southwest England between 2pm on Saturday and 3am on Sunday.
In Wales Cardiff, Swansea and Carmarthen are among the areas that could be affected, while in England Plymouth, Southampton and St Austell could be affected.
The Met Office said: “Rain is expected to develop across south Wales and south-west England during Saturday afternoon, moving slowly east to clear most areas early on Sunday morning.
“Although the most persistent rain is expected over hills, periods of heavy rain are expected almost anywhere. 15 to 25 mm is expected quite widely, with some places seeing 40 to 60 mm of rain, particularly over hills in south Wales and south-west England.
“Strong winds will accompany the rain, with gusts of 45 to 55 mph over hills and around coasts.”
Parts of Scotland, including Perth and Sterling, could also get heavy rain from noon to midnight on Saturday.
The Met Office warned “heavy rain may lead to some flooding and disruption to travel” during this period.
“Rain will turn persistent and heavy at times across the warning area through Saturday afternoon and evening,” it added.
“Accumulations of 20 to 30 mm are expected with 45 mm possible on high ground.
“Given existing saturated ground there is a risk of some localized flooding.”
From Sunday to Tuesday, the Met Office weather outlook states: “Rain across England and Wales will slowly clear Sunday, with blustery showers following to the north.
“Showers in the north die out Monday, with many areas dry Tuesday.