Heavy rain could cause travel chaos across 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 on 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.
The weather warnings on Saturday coincide with train drivers belonging to the Aslef union striking in a long-running dispute over pay, impacting 11 operators.
Companies where no services are expected to run include Avanti West Coast, CrossCountry, Chiltern and East Midlands, while others including Great Western and Greater Anglia say services will be extremely limited.
'No one is listening to us'
Passengers have been urged to plan ahead and check before they travel throughout the whole 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.
Mick Whelan, general secretary of Aslef, said: "We don't want to be in this position, but no one is listening to us.
"Our members did not receive a pay rise during the pandemic, and we are being told that train companies can't afford more than a 2% rise."
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 southwest 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 southwest 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 localised 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.
Which lines are affected?
- Avanti West Coast - No service
- Chiltern Railways - No service
- CrossCountry - No service
- East Midlands Railway - No service
- Great Western Railway - Extremely limited service will operate, starting at 7.30am and ending at around 9.30pm
- Greater Anglia - An extremely limited service will operate
- London North Eastern Railway - An extremely limited service will operate
- London Northwestern Railway - No service
- Lumo - Trains will run only between Edinburgh and Newcastle.
- Northern - No service
- Southeastern - No service
- TransPennine Express - An extremely limited service will operate.
- West Midlands Railway - No service