The spell of Arctic air, which has already forced schools to close, could bring freezing temperatures of -15C.
Tuesday could be the coldest January night since 2010, the Met Office has warned.
Temperatures may drop to freezing lows of -15C in parts of Scotland, according to the forecaster.
Bitter conditions and significant snowfall have already forced schools to shut in Scotland and northern England, and sparked travel disruption.
Yellow warnings for ice and snow are in force in all four UK nations, before possible stormy weather this weekend.
By the end of Friday, more than 40cm of snow may be seen on high ground in north-west Scotland, as wintry weather continues, the Met Office has forecast.
Snow, sleet and rain are expected to continue blanketing northern parts of the UK, though there will be some sunshine. Heavy frosts and freezing conditions are likely across virtually the entire country, with experts warnings of treacherous pavements and roads.
Earlier on Tuesday, the Met Office posted on social media: “Tonight could be the coldest January night for 14 years with temperatures possibly falling as low as minus 15 °C in snow covered parts of Scotland”.
The lowest temperature so far this winter was -12.5C, in Altnaharra, in early December. In January 2010, -22.3C was recorded in the same Scottish Highlands hamlet.
But BBC Weather’s Chris Fawkes said temperatures overnight on Tuesday may only reach around -9C.
Scotland is expected to see the worst of the weather, with up to 10cm (almost four inches) of snow falling on higher ground.
The UK Health Security Agency issued an amber cold weather warning for England this week, meaning the NHS is expected to come under extra pressure and elderly people may be more at risk.
Icy winds blowing in from the Arctic this week have seen temperatures fall 5C to 6C below the average for this time of year.
The Met Office has issued two yellow weather warnings for Wednesday. They are:
- A warning for ice across Northern Ireland from 17:00 GMT to midnight
- A snow and ice warning across parts of northern Wales, England and all of Scotland until midnight
Three more yellow warnings for snow and ice cover most of Northern Ireland, northern Wales, northern England and all of Scotland on Wednesday.
On Thursday, two more yellow snow and ice warnings cover much of the same regions, excluding some eastern areas of England and Scotland and southern parts of Northern Ireland.
The UK is braced for sub-zero temperatures until the weekend, with Tuesday seeing more than 100 school closures in Scotland and dozens in Merseyside.
All schools in Shetland were closed, with more than 50 shut in the Highlands and some sites in Aberdeenshire also affected, though some schools there have reopened.
Northern and eastern parts of Scotland saw the “bulk of the snow” on Monday, with 15cm on the ground at Aberdeen Airport.
Met Office Chief Meteorologist Andy Page has warned of drifting or blizzard conditions.
South of the border, Liverpool Council said conditions had a “widespread impact” on Tuesday, with staff deployed to clear snow around schools, hospitals and transport hubs.
On the trains, National Rail has warned there could be disruption across the network throughout the week.
The cold weather also affected Tuesday’s football schedule with National League matches at Oldham, Halifax, Southend, Woking and York called off, as well as Mansfield versus Forest Green Rovers in League Two.
The cold spell has resulted in automatic £25 payments for households in receipt of certain benefits being triggered in more than 220 postcodes where temperatures are expected to be below zero for a full week.
The assistance with heating costs for eligible households also applies in some parts of Wales and northern and eastern England.
Looking ahead to Friday and beyond, BBC Weather’s Chris Fawkes said: “Snow in Scotland will starts to become confined to hills as temperatures start to rise by an odd degree.
“For those that don’t like the cold weather, there will be a big change this weekend as the weather turns much milder.”
He continued: “This change will be accompanied by very strong, possibly disruptive winds and spells of heavy rain will also return.
“The flooding we had at the start of the year may seem a long time ago for many, but with the ground remaining saturated we’ll have to be watchful of the risk of flooding that any heavy rain may bring.”
At around 21:00 on Tuesday evening, there were also 20 flood warnings – meaning flooding is likely – and 65 flood alerts – meaning flooding is possible – in place in England.
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