Overnight temperatures are expected to fall as low as -5C in Scotland and Northern Ireland, and just above freezing for most of the rest of the UK, the Met Office says.
The drop comes just a week after parts of England and Wales were basking in the hottest October weather in five years, with highs of nearly 26C.
The sharp contrast will be particularly prominent in northern Scotland, which is expected to see snow and strong winds.
The Met Office says there’s also a greater chance of frost over northern England, Northern Ireland and Scotland, especially in rural areas.
“Be prepared for a chill,” says Alex Deakin, Met Office meteorologist.
“It is much colder out there for many, certainly compared to recent mornings and certainly a lot colder than last weekend.
“If you’re heading out this evening then be prepared for those temperatures to be dropping pretty rapidly.”
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Showers will be frequent in the north and west of the UK, with sunshine over eastern England and southern Scotland.
Mr Deakin added: “Temperatures are struggling to get into the teens across the south and many places further north.
“It’s 10C or 11C at best – add on the strength of the wind across northern Scotland and it really does feel cold.”
Temperatures across the south of England are lower than average for this time of year today, even before the expected temperature drop tonight.
Despite the overnight chill, many parts of the UK will have a “fine and settled day” on Sunday with “plenty of sunshine”, said another Met Office meteorologist, Alex Burkill.
Mr Burkill added: “Despite the sunshine, temperatures aren’t going to rise particularly high.
“In fact, for some of us, they could be a little bit lower than Saturday.
“In the south, you’re looking at highs of 12C or 13C and some places in the north are likely to stay in single figures.”
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But Monday morning is expected to have a similar start to Sunday.
The meteorologist added: “First thing Monday morning, you will really notice it – you may need to scrape your windscreen on your cars because it is going to be a chilly, perhaps even frosty start.”
