Drivers in parts of Britain have been urged to stay indoors, after drifting snowfalls closed several main roads.
A lorry jack-knifed on the A11 in Norfolk, forcing the road to close
Snow fell from Scotland to as far south as Suffolk, with mostly eastern parts affected. The Yorkshire Moors had 30 cm (almost a foot) of snow.
Roads were closed by heavy falls, with accidents in North Yorkshire and Norfolk also causing problems.
In Greater Manchester, the driver of a road gritter died when the vehicle overturned.
Emergency teams were called to Saddleworth at 0700 GMT.
Police said it was not clear what had caused the accident.
The Met Office has issued severe weather warnings for Wales, Scotland, Northern Ireland, southern and eastern England and the Midlands.
About 5cm of snow fell on Norfolk.
The A11 road was closed southbound near Wymondham, after a lorry jack-knifed in the early hours of the morning.
A police spokesman said the driver received hospital treatment, but was not badly injured.
The ice and snow is expected to melt on Tuesday, but rain will mean a risk of flooding.
Hailstorms and snowdrifts
Near the North Yorkshire coast, the main A171 road from Scarborough to Whitby and the A169 from Whitby to Sleights were both closed.
The A170 Helmsley to Scarborough road was "very hazardous", with one steep section said to be passable only by 4x4 vehicles.
Police in North Yorkshire and the Scottish Highlands and islands have asked drivers to make only essential journeys.
Drivers across the country were told to allow additional time.
The Highways Agency also warned drivers not to try to overtake gritters and snowploughs on half-cleared lanes.
Later on Monday a change from cold northerly winds to westerly ones is expected to bring mild air and rain from the Atlantic.
That could mean more traffic chaos because of a rapid thaw and the risk of widespread flooding.
Tuesday: Cloudy and misty with a rapid thaw
Christmas Eve: Milder but light rain in some parts
Christmas Day: Fair day and slightly milder again
"Drivers are advised to check for up-to-date information on
weather before they set off, and during their journey," said a Highways Agency spokesman.
The agency also told drivers to travel with warm clothes, food,
water, boots, a torch and spade.
It urged them to remember it can take 10 times longer to stop in icy conditions.
The AA estimated as many as 18 million cars would be on the roads at some point over the holiday period.
Some roads such as the M25 could be packed, with 25% more traffic than normal, it said.
Even more people would travel on Christmas Eve, whatever the weather, as many would have arranged to meet friends and family, it said.
A record 1.8 million people are heading abroad for the festive period.
The Association of British Travel Agents said the most popular winter sun destinations this Christmas were the Canary Islands and southern Spain.
BAA said over the Christmas and New Year period, more than 550,000 people
would be leaving the UK through its seven UK airports, which include Heathrow,
Gatwick and Stansted.