Drivers have been warned not to take routes across the Pennines and airports are preparing for flight disruption as more snowfall is expected to hit the UK on Sunday.
Temperatures could feel as low as -10C in some areas after the cold snap dubbed the “mini beast from the east” sweeps in from Siberia.
Wintry conditions from Saturday are due to continue on Sunday, with the Met Office issuing amber weather warnings for more snowfall in southwestern England, the Midlands, south Wales, and parts of London and the South East.
Highways England has warned drivers to avoid higher-altitude routes through the Pennines, where forecasters say up to 25cm of snow could fall.
"It's going to be a very, very cold start, with a widespread frost and ice around as well,” Met Office meteorologist Mark Wilson said.
"And we have plenty of snow showers to start the day. These will be heaviest and most frequent across southwestern areas, so we're talking parts of Wales and the south west of England in particular.
"Across some southwestern parts the snow will continue throughout the day. We're looking at some fairly large snowfall totals."
Meanwhile, airports have warned passengers to check with airlines for potential delays and cancellations caused by the cold snap.
The airport said "extreme wintry weather" is expected, adding: "In preparation, we've worked with our airlines to consolidate Sunday's flight schedule, moving passengers on to fewer flights.
"This decision enables us to provide more certainty to passengers around departing flights, whilst ensuring safety and service standards despite the challenging conditions.
"Our teams will be working throughout the night to keep our runways and taxiways operational and we will be closely monitoring the weather forecasts with our on-site Met Office.
"As always, we advise passengers to take care when travelling to the airport and ensure they check their flight status with their airline before coming to Heathrow."
Gatwick Airport tweeted on Saturday night: "Gatwick is currently operating a normal service, however the weather is predicted to deteriorate and all passengers are strongly advised to check with their airline for the latest flight information before leaving for the airport."
Stansted and Luton airports also advised passengers to check with their airlines.
The M25 was gridlocked on Saturday between junctions 17 and 18 anti-clockwise after an air ambulance was called in to a crash.
A collision also closed the westbound junction of the M4 in Wiltshire between junctions 14 and 15.
Richard Leonard, Highways England's head of road safety, said: "Our gritter drivers will be out treating our roads around the clock but it is still important to drive to the conditions when snow is forecast.
“Drivers should plan their journeys, monitor weather reports and pack a snow kit of blankets, food, water and a shovel if they really need to travel."
Additional reporting by Press Association
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