An Arctic blast will send temperatures as low as -12C in parts of the UK this week after an unseasonably mild weekend.
The hillier parts of Scotland are braced for the coldest weather, although forecasters expect widespread sub-zero conditions in rural areas on Monday and Tuesday.
The wintry start to December follows a weekend when parts of south-west England enjoyed temperatures as high as 16C on Sunday.
The cold snap is expected to cause some travel disruption on higher ground, with ice, frost and fog forecast for the first half of this week, meteorologists said.
Met Office forecaster Marco Petagna said the sudden change in temperature was due to icy winds from the Arctic blowing into the UK on Monday, replacing milder air from the south.
He said: “It could be -12C on Monday night in the higher ground of Scotland where we’ve got snow cover. It’s not unusual, given it’s winter now.
“Things are turning colder from the north, certainly across the higher ground of Scotland we will see some wintry showers developing, staying milder further south, although the colder air will move southwards during Monday.”
Petagna said parts of Wales could record temperatures as low as -5C and higher areas in England, such as the Peak District, were expected to reach -4C on Monday night.
He added: “There could be widespread frost and icy patches in places, and could be the odd mist or fog patch on Tuesday morning.”
The northern half of the UK could have snow on Tuesday as rain moves into colder air on higher ground.
Temperatures will rise across the UK on Tuesday afternoon thanks to warmer air from the west, forecasters said.
By midweek, scarves and gloves will be swapped for umbrellas as rain moves across the country, turning the first sprinklings of snow to slush.