Snow is predicted to fall in northern England and Ireland and possibly in the Midlands and Wales, after around eight inches (19cm) came down across parts of Scotland.
Temperatures are set to plummet to as low as -5C tonight north of the border, as the big chill moves south, following one of the warmest-ever Marchs.
On Monday last week, it was 23.3C in Cromdale in Moray, making it the hottest March 26 since records began.
And last Tuesday it was 23.6C in Aboyne, Aberdeenshire , marking a new high in Scotland for the month.
But the situation is markedly different now, as about 4in (10cm) of snow blanketed most areas of northern Scotland overnight, with accumulations of 7in (18cm) in Aviemore in the Highlands and temperatures of close to -2C in Glen Ogle.
Nine Belgian hillwalkers, including six children aged between 14 and 16, were rescued by helicopter after heavy snowfall in the Ben MacDui area in the Cairngorms , Grampian Police said.
Police said all nine people were in "very good spirits" and that none had to
go to hospital.
Royal Navy rescue helicopter navigator Lieutenant Angela Lewis said: "Wind speeds on Ben Macdui when we arrived on scene were probably in the region of 50 knots and wind chill around -35C, so it was quite unpleasant - very, very cold."
And the wintry weather cut off power supplies in some parts of the country with 6,000 Scottish Hydro customers still without power this afternoon.
The company said it hopes to have its customers reconnected by early evening.
Scotland's central parts were also hit by the snowy conditions, which will gradually creep into northern England, parts of Wales and the Midlands today.
The Met Office has issued a yellow severe weather warning for people to "be aware" of snow in Wales, north-west and north-east England, Yorkshire and Humber and the Midlands.
In Scotland, there is a warning of ice on the roads in the Highlands, Grampian, Strathclyde, and Central, Tayside and Fife regions, as well as southwest Scotland and Lothian Borders.
However, forecasters said the cold snap, which has been caused by an Arctic weather front bearing down on the UK, will be short-lived with temperatures recovering on Thursday.
London and the southeast were expected to escape the snow with temperatures peaking at around 14C today before dropping to around 8C on Wednesday.
Sky weather presenter Isobel Lang said the snow in Scotland was "most likely to settle over the higher ground, with more slushy accumulations at lower levels".
She added: "However, some higher routes could be affected by settling snow for a time, bringing dangerous driving conditions.
"Sunshine will become more widespread across Scotland this afternoon, as the wintry weather clears from the Borders, but there will still be a few snow showers around coasts. It will turn icy tonight but stay largely clear.
"For Ireland, northern England, the Midlands and Wales, expect rain at first, but conditions will turn increasingly wintry as that cold air sinks south and the northeast wind sets in."
Lang added: "Snow will fall over higher ground in northern England and Ireland by the end of the morning, affecting higher Pennine routes.
"This risk is then set to spread into the Midlands and Wales by the end of the day and it will have turned much colder by this stage.
"Snow is most likely over higher parts here, perhaps extending into the West Country or South West at times. For southeast England it will remain less cold through the day and night although there will be showery rain at times - temperatures in London should drop to 6C tonight.
"On Wednesday the cold air will linger across southern Britain with the northeast wind and outbreaks of rain, sleet and hill snow.
"The snow is most likely over the Peaks, into the hillier parts of South Wales and the southwest moors.
"The remainder of the week will see less cold air spreading in from the Atlantic on a west to north-westerly wind - so temperatures will rise again but expect a fair amount of cloud into the Easter weekend with rain at times."