Bitterly cold temperatures will feel as low as minus 10C as the wintry snap dubbed the "mini beast from the east" keeps its grip on the UK.
The Met Office has weather warnings in place for more snow and ice, with some areas likely to see up to 25cm of snow on Sunday.
Flight passengers are advised to check with their airline for the latest flight information, while drivers are warned to take care.
Met Office meteorologist Mark Wilson said: "It's going to be a very, very cold start, with a widespread frost and ice around as well.
"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."
Mr Wilson said people could see 10-15cm of snow, and up to 25cm over higher ground.
He said snow showers in other parts of the country are expected to ease throughout the day, with some spells of sunshine possible.
"It will stay windy, and it will stay very cold indeed. Temperatures barely above freezing, and in the wind it will feel like it's well below freezing," he said.
Mr Wilson said it will feel as low as minus 8C or minus 10C in places, particularly across southern areas.
Heathrow 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 Airport and Luton Airport also advised passengers to check with their airlines.
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.
"Make sure you keep your distance and reduce your speed if you need to travel because, even in conditions that seem normal and when the snow is not settling, it can be slippery if ice patches have formed, or where fresh salt has not been worked into the carriageway.
"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."
The M25 was gridlocked on Saturday between junctions 17 and 18 anti-clockwise after an air ambulance attended a crash, while a collision also closed the westbound junction of the M4 in Wiltshire between junctions 14 and 15.
Meanwhile, 11 people were rescued or made their own way to safety after two rowing boats and an accompanying safety boat capsized in the River Stour in Bournemouth.
Local media reported that most of the group were teenage girls, along with one adult in the safety vessel.