The Met Office has issued severe snow and ice warnings as the Troll of Trondheim hits the UK with an Arctic blast.
The pre-Christmas cold snap will see temperatures sink as low as minus 10 overnight - as well as snow of up to 10-15cm in parts of Scotland.
Forecasters say biting winter air will move in this evening, with the UK Health and Security Agency issuing a cold weather alert recommending people warm their homes to at least 18C (64.4F).
The Met Office said on Twitter: "If you haven't already, it's time to dig out the winter hats, gloves and scarves."
The weather agency has put multiple yellow warnings in place, including for ice across western and southwestern England, Wales and Northern Ireland from 5pm today until 6pm tomorrow.
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While a similar warning for snow and ice is in place for much of Scotland from midnight today, and now extended until midday on Friday.
It adds: "Some drifting and blizzard conditions are likely in the strong northerly winds.
"In addition, ice is likely to form on untreated surfaces, mainly where some of the showers fall as rain or sleet on some coasts."
Wintry showers could also fall on high ground in northeast England, North Wales, the North York Moors and Northern Ireland.
Forecaster Oli Claydon said: "Day to day we are in cold conditions already with that north-easterly flow, but conditions are set to get colder through the week, with the worst showers across northern Scotland but also bringing much colder conditions across all the UK.
"Overnight lows of minus 10 to minus 11 (12.2F) in areas where we do get snow in those rural parts of Scotland, with temperatures down to minus six (21.2F) in rural England.
"Cold conditions to remain through the weekend. Signs of warmer weather moving in from the southwest from Tuesday next week.
"But the signs are that the cold weather will hold in the north of the UK, so there will be a split between north and south."
Homeless people in London are to be sheltered after the Severe Weather Emergency Protocol (SWEP) was activated for the first time this winter to provide emergency accommodation for rough sleepers.
