Britons have been warned to expect widespread sub-zero temperatures and icy, treacherous conditions caused by Sunday's snowfall.
The Met Office said the mercury could dip between minus 4C (24.8F) and minus 6C (21.2F) going into Monday, with parts of the Midlands told to brace for minus 10C (14F).
An amber weather warning for snow is in place covering an area from Nottingham to Stoke-on-Trent, meaning travel disruption and power cuts are likely.
There is also a risk that some rural communities could be cut off.
Parts of the Midlands could record 15cm (6in) of snowfall by the end of the day, with the snow expected to remain on the ground well into next week.
Meteorologist Simon Partridge said: "The band of snow that came through overnight stalled over the Midlands and will then track its way back over south-east England."
He added: "Our main focus is that it is going to be a really cold night, particularly across central and southern parts of the UK – so the whole of Northern Ireland, all of Wales, most of the southern half of England from the Midlands and southwards."
Mr Partridge added: "Temperatures are expected to dip to between minus 4C (24.8F) and minus 6C (21.2F) quite widely tonight and where we get that snow lying in parts of the Midlands we could see one or two places where temperatures are as low as minus 10C (14F)."
Even in areas with lighter snowfall, a widespread frost is forecast, and conditions are expected to be hazardous including spots of ice on the roads.
Tonight will be another very cold one, especially where we have snow cover. Why?
At night, snow is very effective at emitting longwave radiation back to space, which decreases the temperature at the surface
Rural parts of England/Wales could see lows approaching -10 to -12 °C pic.twitter.com/opszzTngNd
— Met Office (@metoffice) January 24, 2021
Large parts of the country woke up to snow and ice on Sunday morning, with even London getting a dusting of white.
The snow brought widespread delight, allowing Britons to forget the pandemic, and the hashtag "Narnia" was trending on social media.
Is it Battersea Park, or is it Narnia? ❄️ 🦁 ⚔️ 👑 pic.twitter.com/Gw4E6djf7z
— Rhodri Morgan (@RhodriDEMorgan) January 24, 2021
But it was not without its challenges.
Buckinghamshire and Milton Keynes Fire and Rescue Service drafted in its Bobcat utility vehicle to clear snow at the Adams Park vaccination centre in High Wycombe so the rollout was not interrupted.
Firefighters used the @Bucksfire Urban Search and Rescue Bobcat utility vehicle to complete the clearing of snow at Adams Park vaccination centre in High Wycombe today#WorkingTogetherpic.twitter.com/b4RqvAToUR
— Bucks and MK Fire (@Bucksfire) January 24, 2021
Elsewhere, the people of Seer Green in Buckinghamshire were called upon to rescue an ambulance stuck at the bottom of a hill in thick snow.
The South Central Ambulance Service tweeted: "A huge thank you to the people of Seer Green in South Bucks who came out in force to help one of our crews make it up a hill in thick snow at the bottom of Orchard Road/Bottom Lane earlier today.
"Great to see such community spirit and support for our NHS! Thank you guys!"
A huge thank you to the people of Seer Green in South Bucks who came out in force to help one of our crews make it up a hill in thick snow at the bottom of Orchard Road/Bottom Lane earlier today.
Great to see such community spirit and support for our NHS! Thank you guys! 👏👏👏 pic.twitter.com/nLtWkWaNZ7
— South Central Ambulance Service (@SCAS999) January 24, 2021
In Wales, four vaccination centres were forced to close because conditions were so treacherous.
The Cwm Taf Morgannwg health board said centres in Bridgend, Rhondda, Abercynon and Merthyr Tydfil would reopen on Monday, and that Sunday's appointments would be rescheduled.
The board said no vaccines had been wasted, while 4x4s were being used to make sure doses still got to care homes over the course of Sunday.
There are five yellow weather warnings in place for snow and ice covering the southern part of England, eastern Scotland and Northern Ireland.
Icy patches on untreated roads and pavements are likely, with journeys by road and rail likely to be affected.
But the Met Office said by next week temperatures are due to jump back up.
Mr Partridge said by Wednesday the south of England would likely see between 10C (50F) and 12C (53.6F).
"It's a bit of a rollercoaster from cold and wintry conditions to wet and windy ones," Mr Partridge said.
"We are keeping an eye on rainfall totals because there are some areas that are very sensitive to rainfall and there is further possible flooding as we go through the second half of the week."