Most parts of the UK are due to see heavy snow later, bringing disruption to roads, rail and air travel.
Snow has fallen in south Scotland, the West Midlands and north-east England, with the Met Office issuing severe weather warnings for much of the UK.
Forecasters say 5-10cm (up to 4in) will fall in central, eastern and southern England, including London, with the heaviest falls expected after dusk.
BAA has cancelled one-third of Sunday’s flights from Heathrow Airport.
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.
A list of which flights will operate will be available on the Heathrow website from around 1800 GMT on Saturday.
Passengers at London Heathrow, Gatwick and Stansted are advised to contact their airline for more information.
Air travel has 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 says it will allow passengers booked on Sunday flights to re-book for journeys between Monday and Thursday.
A spokeswoman said the airline would assess flying conditions throughout the weekend.
Sporting fixtures have been badly affected, with many football and race meetings cancelled.
The Met Office has issued amber “be prepared” warnings for snow and ice across most of Britain, with yellow “be aware” alerts for the Highlands and Northern Ireland.
Light snow flurries began falling in Manchester around midday, leaving a covering over rooftops.
The icy conditions prompted North West Ambulance Service to ask people to stay indoors following an increase in fall-related 999 calls.
Forecasters say there is potential for transport disruption and warn motorists to take shovels, warm clothes and fully charged phones.
The Highways Agency has extended its amber alert until 09:00 GMT on Sunday, meaning there was a “high probability” of severe snow affecting the road network.
BBC forecaster Louise Lear said a band of wet weather would fall as snow as it moved south and east, with the bulk falling after Saturday teatime.
“We could see five to 10cm of snow quite quickly across central and eastern England, and maybe as much as 10 to 15cm on higher ground.”
Ice would become an added danger on the roads by Sunday morning, she added.
The RAC’s Kevin Andrews said the situation was likely to deteriorate throughout Saturday and urged drivers to stay at home where possible.
“It looks like we’re going to get a dangerous cocktail of driving conditions this weekend, with heavy snow and sub-zero temperatures making the roads extremely treacherous.”
The motoring organisation was attending 70% more breakdowns than usual. An AA spokesman said it was dealing with around 1,500 vehicles per hour on Saturday morning and was expecting double the normal level of callouts.
February, traditionally the coldest month of winter, has seen daytime temperatures plunge four or five degrees lower than average over the past few days.
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A temperature of -12.4C (9.7F) was recorded in South Newington, Oxfordshire, overnight – 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 have been 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.