Heavy snow is falling across much of the UK, disrupting flights and leading to calls for drivers to take more care.
Snow hit southern Scotland and parts of Wales before moving south-east through England.Met Office severe weather warningsremain in place across the UK.
Forecasters expect 5-10cm (2-4in) to fall in central, eastern and southern England, leaving icy roads on Sunday.
Luton airport in Beds has closed its runway due to snow while Heathrow is cancelling some Sunday flights.
Heathrow operator BAA said one-third of Sunday's approximate 1,200 flights would be cancelled.
There were some delays on Saturday night due to visibility problems.
On the roads, police said as many as 100 vehicles were stationary on the M40 between Junction 4 High Wycombe and Junction 9 Bicester for several hours, however traffic is now starting to clear as the snow is easing off.
Traffic is slow moving on the M25 between J18 Chorleywood and J19 Watford because of snow.
In other developments on Saturday:
- North West Ambulance Service asked people to stay indoorsfollowing an increase in fall-related 999 calls
- There were warnings of icy roads after5cm of snow fell in parts of Scotland
- In Wales,treacherous driving conditions were reportedin the Brecon Beacons and heads of valleys
- Transport for London prepared grittersto put more than 10,000 tonnes of salt on the capital's roads
- Saturday's football programmewas badly disrupted
- Thesea froze in places, including Dorset's Sandbanks resort
- The AA warned motorists to pack shovels, warm clothes, snacks and fully charged phones
Heathrow's chief operating officer Normand Boivin said the decision to introduce a revised flight schedule before snow had fallen was taken in an effort to minimise disruption to passengers.
Richard Scott, from BAA, later said because Heathrow runs at capacity, there was no slack in the system to deal with the reduced number of flights that can take off and land in bad weather.
He added: "If you can imagine you're driving, you have to drive slower, leave more space for the car in front in fog, strong winds, snow. It's the same with aircraft.
"If we don't proactively cancel flights, the delays build and build and build and it's much worse for passengers."
Flights have been diverted from Luton airport and three departing flights have been cancelled.
Stansted airport in Essex closed on Saturday night for a period to clear snow from its runway while Birmingham Airport said it was clearing snow from the airfield and runway.
Leeds Bradford Airport remains open but has warned of delays or cancellations.
On the London Underground, the weather caused the entire Jubilee Line to be suspended and parts of the Metropolitan, Northern and Piccadilly lines were suspended.
Earlier on Saturday, the Highways Agency extended its amber alert of a "high probability" of severe snow to 09:00 GMT on Sunday.
The RAC's Kevin Andrews said heavy snow and sub-zero temperatures would create "a dangerous cocktail of driving conditions" and urged drivers to stay at home where possible.
Among places where traffic problems were reported were Stoke on Trent and East Yorkshire, where council workers were said to be battling to keep open roads across the Wolds.
Air passengers at London's Heathrow, Gatwick and Stansted airports have been advised to contact their airline for more information.
Travel had already been disrupted across the whole of Europe - flights from Rome and Amsterdam's Schiphol airports to UK destinations, and Saturday morning flights to Amsterdam from Birmingham, Manchester and Heathrow were cancelled or delayed.
Due to the worsening conditions, British Airways said it would allow passengers scheduled to fly on Sunday to re-book for journeys between Monday and Thursday.
The Met Office issued amber "be prepared" warnings for snow and ice across most of Britain, with yellow "be aware" alerts for the Highlands and Northern Ireland.
Forecasters said a band of wet weather had turned to snow as it moved south and east across mainland Britain.
The BBC's John Hammond said south-east England and East Anglia was set for a "very snowy night".
"It will continue to pile up. We can expect as much as five to 10cm of snow, and over the high ground we can expect more," he said.
"With temperatures close to freezing nationwide, it's not just snow we've got to worry about - it's ice as well. It will be a major concern up and down the country first thing in the morning."
February, traditionally the coldest month of winter, has seen daytime temperatures plunge four or five degrees lower than average over the past few days.
A temperature of -12.4C (9.7F) was recorded in South Newington, Oxfordshire, on Friday night - the lowest this season, the Met Office said.
The cold conditions are likely to continue into the early part of next week.
The Local Government Association said motorists were being advised to check the latest weather and gritting updates on council websites and "gritter Twitter" feeds.
British Gas said its engineers were on stand-by in all-weather 4x4s, and the Department for Transport said Britain's salt stocks stood at more than 2.4 million tonnes - a million more than last year.
The charity Age UK said it was a dangerous time for older people with low temperatures raising blood pressure, which put people at greater risk of heart attacks and strokes as well as the risk of flu.
In Championship football, Saturday's games at Portsmouth and Doncaster were postponed.
In League One, frozen pitches meant only matches at Carlisle, Huddersfield, Sheffield Wednesday and Wycombe survived. All League Two fixtures bar Plymouth's with Southend were postponed.
Falkirk's Scottish Cup match at Ayr was called off and, of the league programme, only the Third Division games at East Stirling, Montrose and Queens Park went ahead.
Horse racing fixtures at Ffos Las, Sandown and Wetherby on Saturday were called off. Sunday's meeting at Kempton will be subject to an inspection at 08:00 GMT.
Wolverhampton's all-weather meeting was abandoned after four races because of snow.
The weather warnings in Britain follow a freeze across many parts of Europe, where temperatures have fallen as low as -30C in some parts, and scores of people have died, including more than 100 in Ukraine.
Freezing weather has led to a shortage of vital Russian gas supplies to several countries, and Italy has seen its coldest week for 27 years.