A severe weather warning was in force last night as forecasters predicted the first “Siberian” blast of the year would bring icy roads and snow across the Westcountry.

The Met Office said up to two inches (5cm) of snow could fall on higher ground in areas such as Dartmoor and Exmoor with a widespread “dusting” expected in Devon, away from the south coast.

Drivers were urged to take care as temperatures were set to plunge to -5C (23F) overnight leaving icy roads potentially covered with a slushy covering by the morning rush hour.

Craig Snell, a Met Office forecaster, said: “We have got a band of rain slowly moving towards us and coming into contact with a huge area of high pressure from Eastern Europe – known as a Siberian high.

“This will fall as rain in Cornwall but will turn to sleet in snow in Devon, which will probably start to melt in the morning at lower levels but could last longer on the moors.

“Untreated roads could be a bit icy in the morning but trunk roads should hold up well, though with pockets of disruption.”

Police last night said there had been reports of snow causing some problems in Princetown, Dartmoor, and the Lynton area of Exmoor.

Much of the country, including the South West, has been enjoying one of the mildest winters in 350 years. But the relatively balmy temperatures are likely to fade away as the Met Office said there was only a one-in-three chance of warm westerly winds returning soon.

The Exeter-based Met Office put the region on amber alert for snow last night.

Chief forecaster Chris Tubbs, said: “Looking further ahead into February, there is uncertainty over how this finely balanced situation will play out, with colder conditions more likely to dominate but milder weather could still return.”

In Devon and Cornwall a “cold dry week” was predicted with daytime temperatures no higher than 4C (39F) and clear skies expected on Wednesday.

Jonathon Jones, garden director at Cornwall’s Tregothnan estate, said: “We are ready for colder temperatures but so far we have not even had a single frost this year, which is very unusual.”

The cold snap is likely to provide the first test of Devon County Council’s scaled-down gritting operation.

The authority, which relegated 57 miles (90km) of roads to the secondary salting network as part of its winter weather strategy, said trucks began treating about 1,700 miles of the main salting network at 6pm and midnight on Friday.

Residents in the hilltop village of Bridford, on Dartmoor, have accused the council of leaving key workers, school children and the elderly isolated and putting lives at risk.

Councillor Stuart Hughes, Devon County Council cabinet member for highways, said: “The mix of rain, sleet and freezing conditions means that road users must be aware that conditions on the county’s roads may be icy, even on treated roads.”