Heavy snow and ice cause disruption across the UK
Heavy snow falls across Scotland, Wales and Northern Ireland
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Several UK airports are closed and train services have been cancelled due to heavy snowfalls in many areas.
Police in Northern Ireland - where conditions are said to be the worst in 25 years - and parts of Scotland are warning against non-essential travel.
At least 800 schools in Wales, 500 in Scotland and 700 in Northern Ireland are closed.
More snow is forecast later in western areas and on Saturday, 10in (25cm) is possible in Wales and southern England.
The Met Office has issued severe weather warnings for heavy snow - up to 8in - in many places, including Orkney and Shetland, Highlands and Outer Hebrides, Grampian, Northern Ireland, Wales, the Midlands, and the south-east and south-west of England.
Strong winds will cause the snow to drift in some areas and ice could be a problem anywhere in the UK, with temperatures struggling to get above freezing.
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Winter weather has caused some disruption
What's the weather in your area?
BBC Weather The BBC Northern Ireland weather centre said the current snow situation was probably the worst widespread fall in about 25 years.
In 2000, similar amounts of snow fell in about four hours, but were restricted to eastern counties.
By Saturday, a second band of heavy snow is forecast to affect southern England and Wales, with 2-4in likely in many places and 8-10in possible in some others. Forecasters say there is some uncertainty about how far north it will reach.
Problems caused by the weather include:
Several major roads - including a 50-mile (80km) stretch of the A9 in Scotland - and many minor roads are blocked by snow
Police say conditions have been "particularly atrocious" in Caithness, Sutherland and Easter Ross, and some motorists had to be rescued from their cars after becoming stranded
Flights are suspended at Exeter, City of Derry, Belfast City and Belfast International airports
London City, Norwich, Aberdeen, Inverness and Isle of Man airports have reopened after earlier closures, but flights are subject to delays and cancellations. Disruption is possible at other airports, including Gatwick, Heathrow and Cardiff
ScotRail, and Arriva and Virgin train services in Wales are experiencing delays and cancellations
Southern, Southeastern and South West trains have cancelled some services on Saturday in anticipation of more bad weather
An urgent appeal is being made for blood donors, particularly those who are O negative, as stocks are running low
A number of weekend football fixtures have been cancelled and Friday's race meetings at Uttoxeter and Ascot have been also been called off
Friday was due to be the last day of term in many schools, but hundreds have closed their doors early because of the weather.
'Absolutely awful'
New Scottish Transport Minister Keith Brown has insisted Scotland is "very well prepared" and the authorities are doing all they can to keep trunk roads open.
Continue reading the main story WEATHER AND TRAVEL INFO
Get the latest on travel problems and school closures via your local website
Check if snow is forecast in your area at BBC Weather
Details of motorway and local road closures and public transport disruption are available at BBC Travel News
For advice on handling difficult driving conditions, see the Highways Agency website
For information about severe weather warnings, see the Met Office website
For information about staying healthy in the cold weather, see the NHS Winter Health website
Mr Brown's predecessor Stewart Stevenson resigned after his department was criticised for its handling of the problems caused by the previous bout of severe weather.
Breakdown organisation the AA said it had received 7,500 callouts by midday - about 40% more than a normal Friday close to Christmas.
Spokesman Gavin Hill-Smith said conditions in some areas were the "worst imaginable, even for experienced drivers", with Aberdeenshire worst hit.
And he added: "The trouble is that the closer we get to Christmas, the greater the pressure on people to travel - Christmas shopping, visiting family and friends and we still have a week of Christmas parties to go - but we just recommend that people try to be flexible and willing to adapt their travel plans at short notice.
"The bottom line is that if road conditions are exceptionally bad, you just have to heed the advice on when it is safe to travel."
The Association of Train Operating Companies said it would run "ghost trains" to help keep lines open and had hundreds of ground staff on duty to keep key junctions and points clear of snow.
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Around the UK
Travel problems as snow hits north
Snow causing chaos to Wales
Snowfall in NI worst in 25 years
Energy Minister Charles Hendry warned on Thursday that more bad weather over Christmas could lead to "very serious" shortages of domestic heating oil.
But a spokesman for Prime Minister David Cameron has played down the suggestions, saying: "I don't think that's going to happen at all.
"The position at the moment is that there is sufficient fuel. The problem is getting it around the country."
Heavy snowfall during the past few weeks has caused huge disruption, especially in Scotland and north-east England.
Airports, roads and schools were closed, and companies are now warning of a backlog of deliveries which may not reach customers before Christmas.
The Royal Mail is putting on 7,000 delivery rounds this Sunday in a bid to ease the problem.
