A blanket of snow will cover Britain on Friday with up to 12in (30cm) falling on higher ground, amid fears the latest bout of wintry weather will bring the transport network to its knees.
Temperatures are expected to remain below zero in large parts of the country on what is predicted to be a particularly "messy" day for travel.
Blizzards and strong winds are set to make conditions treacherous on the roads and motorists were warned heavy snow would arrive before the morning rush hour.
South West Trains has already announced changes to its services and will be operating a revised timetable on a number of routes on Friday.
The Met Office has issued a "red" severe weather warning for heavy snow in upland parts of southern Wales. Amber alerts were in place for the Midlands, London and the South, the east of England, the North West and Northern Ireland.
Dumps of nearly 12in (30cm) are possible over the hills, a Met Office spokesman said. The arctic conditions are expected to set in overnight, hitting Wales and the South West before pushing across the UK.
Laura Caldwell, a forecaster for MeteoGroup, the weather division of the Press Association, said 4-6in (10-15cm) is predicted to fall quite widely.
"Parts of the West Country, Wales, the West Midlands and central and southern England are going to get quite a bit of snow tomorrow," she said. "There will be quite a few centimetres in the morning with up to 10cm falling through the day.
"That snow will carry on, pushing north and east so it will cover all of England by the end of the day and even parts of southern Scotland."
While Wales and the Midlands are expected to see the worst of the snow, London and East Anglia are likely have accumulations of around 4in (10cm).