Britain is set to be hit with a -4C cold snap replaces summer temperatures in just one week.
There is an increasing possibility of snow as overnight temperatures fall to as low as -2C in northern rural spots.
Cloud and rain will help keep mercury slightly higher in parts of the south of England.
A north-south divide is expected to take hold, with Scotland bracing for particularly cold conditions.
WATCH NOW: The Met Office's Thursday forecast
Met Office meteorologist Alex Burkill suggested summer could become winter within just one week.
He forecasted temperatures will be "very cold" over the weekend.
Mercury could drop to as low as -4C on Saturday night.
Burkill said: "That's when we're likely to see temperatures several degrees below freezing Saturday night. Sunday night we could be looking at lows of around 3 -4C pretty widespread, so frost is likely.
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A couple walk in the mist at Dungannon Park, Northen Ireland
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"A real drop in temperatures spreading from the north as that cold air plunges its way southwards as we go through this week."
Brent Walker, deputy chief meteorologist at the UK’s national weather service, added: "As we head through second half of this week cold air will push southwards across the country and there is a risk that showers over mountains of Scotland could turn wintry.
"By the weekend we expect all regions of the UK to be in the cold airmass and overnight frosts are possible.
"With high pressure continuing to dominate our weather early next week, it will start largely fine, settled, and cool by day, with cold nights and a risk of rural air frosts in places.
"Any early morning mist or fog should clear quickly and there could be a few showers possible around some coasts at times."
A yellow weather warning has been issued for Thursday and Friday
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The change in temperatures comes just days after Britons were basking in the sunshine during a last-minute Indian summer.
Writtle in Essex saw mercury exceed 25C for four successive days, with the Met Office confirming the last time this happened in October was back in 1959.
Britons are also facing a yellow weather warning today and tomorrow.
The rain alert is impacting the East Midlands, East of England, London, the North West, the South East, South West, West Midlands and Wales.
Six flood warnings have also been issued across Britain.
