Parts of the UK are set to face localised blizzard conditions, winds of up to 80mph and a yellow ice warning as the UK Health Security Agency urged people to check in on older neighbours.
The Met Office issued a yellow ice warning for most of northern England, running from 2am to 11am on Tuesday.
Icy conditions and potentially heavy snows are possible from midnight on Tuesday until lunchtime on Thursday, forecasters said.
There are also snow, ice and wind warnings in parts of northern Scotland.
It follows what the Met Office described as “an exceptionally mild spell” that saw the warmest New Year’s Day in the UK since records began.
Tuesday will be “noticeably colder across the whole of the country”, the Met Office said, with temperatures peaking at 8C (47F) or 9C (48F) in the South and about 4C (39F) or 5C (41C) in Scotland.
In England, the colder temperatures might bring snow over the Pennines and North York Moors, but it will be “pretty wet stuff”, forecaster Simon Partridge added.
However, in Aberdeenshire and northern and eastern Scotland, winds of up to 80mph could create “localised blizzard conditions”, with the Met Office issuing warnings of snow and ice for travellers.
On Wednesday night, temperatures are forecast to drop to -1C (30F) in London – with 0C (32F) in Belfast, 1C (33F) in Cardiff and -3C (27F) in Edinburgh.
During the day, the west of the country is likely to be “dry and bright” as the winds drop, with few showers, but Thursday will see a “band of rain” move in, with icy temperatures overnight, Mr Partridge said.
Towns in the south could face temperatures of -4C (25F) on Thursday morning.
Agostinho Sousa, a consultant in public health medicine at UKHSA, warned that cold weather can have a “serious impact on health, particularly for older people and those with heart and lung problems” as he urged people to check in with older neighbours as temperatures fall.
“It’s important to check on those who are more vulnerable to cold weather, including older neighbours or relatives – especially those living alone or those who have a serious illness,” he said.
“Remind them to heat their home to at least 18C (64.4 F) and to keep up to date with the forecast.”
The call to keep homes heated comes as energy bills continue to rocket, something which has been described as a “nationwide crisis” that has led several suppliers to collapse.
UKHSA said that for people struggling to afford heating bills, Simple Energy Advice provides “free advice on energy efficiency and national grants that are available to help keep you warm this winter”.
It added: “If people can’t heat all the rooms they use, it’s important to heat the living room during the day and the bedroom just before going to sleep.
“Wearing a few thin layers is better at trapping heat than wearing one thick layer. Having plenty of hot food and drinks is also effective for keeping warm.”