Forecasters have issued a "danger to life warning" for fog while more than 30 flood alerts are in place in the UK ahead of Christmas Day.
Freezing fog is expected to cause disruption across all of eastern and central England, where a yellow weather warning has been issued from 3pm on Monday until 11am on Tuesday.
The Met Office warned that bus and rail journeys could be delayed and flights may also be affected.
Some 32 flood alerts were also issued in England on Monday, largely in low-lying areas.
Temperatures plummeted to -4C in parts of Scotland on Sunday night but remained in double figures in south-western parts. It is expected to be mild for most of the country on Christmas Day, with colder conditions further north.
Christmas Day is looking “largely dry, with some cloud and the odd brighter spell,” the Met Office said.
A forecaster told the Standard: “On Christmas Eve, we can expect fine weather with spells of sunshine in most of the UK.
“It will be coldest where the fog doesn’t lift in places like Lincolnshire, but temperatures will be average and it’s a similar picture for most of the country.
“Tomorrow will be largely dry, with some cloud and the odd brighter spell.
“It’ll be fairly bland and grey, but there’s no snow likely because there will be no precipitation.
“We could see the odd spit and spot of drizzle over hills, such as in southern Scotland and North West England.”
On Boxing Day, there is a chance of rain in northern Scotland, but elsewhere should remain cloudy and dry, with a band of rain is set to edge southwards next week.
Forecasters said temperatures could get colder into the New Year and will fall further towards the middle of January.
