The Met Office has issued a red warning for extreme weather in Scotland, as a second day of heavy snow causes major disruption across swathes of the UK.
The central belt, from Edinburgh to Glasgow, is covered by the alert from 15:00 GMT, meaning the risk to life is likely, heavy snow is forecast and services will be cancelled.
Amber warnings are also in place elsewhere in Scotland, northern and eastern England, and the East Midlands.
Police advise staying off the roads.
The snow has forced the closure of hundreds of schools from Cornwall to northern Scotland, after overnight temperatures dropped to almost -12C (11F) in some areas.
A number of flights have been cancelled and rail operators have warned disruption will continue throughout Wednesday.
Rural communities could also be cut off as power cuts and mobile phone network interruptions are expected.
The red weather warning means that extreme conditions are expected and immediate action should be taken.
The forecast from the Met Office, which lasts until 10:00 on Thursday, said: “Roads will become blocked by deep snow, with many stranded vehicles and passengers.
“Long delays and cancellations on bus, rail and air travel are expected [and] some communities could become cut off for several days.”
It also warned of “long interruptions to power supplies and other services”.
As many as 400 schools in Scotland have shut their doors, with hundreds more closing in England – including in Kent, Sussex, Surrey, Essex and Yorkshire – and some 25 closures in Wales.
Airports including London City, Glasgow and Newcastle have suspended flights as they work to clear snow from their runways. More than 100 flights at Heathrow have been cancelled.
National Rail is also warning that services could face major disruption, with the Stansted Express already cancelled between Liverpool Street and Bishop’s Stortford, and Virgin Trains’ route between Carlisle and Scotland closed with no replacement bus services.
Hospitals in Grimsby, Scunthorpe and Goole have cancelled all outpatient appointments because of the weather, while Harrogate Hospital has asked staff who can walk to work to go in to cover shifts.
In other travel developments:
- British Airways says flights are likely to be affected throughout the week and warns that for “several hours during the worst of the weather” on Wednesday, flights will be reduced “proactively” at Heathrow
- Gatwick Airport says it is running without delays, but has asked passengers to check with airlines before travelling
- Some flights out of Edinburgh have been cancelled and more than 50 departures have been cancelled from London City Airport
- Delays and cancellations on the c2c line from Essex into London were worsened by freezing doors stopping passengers getting on and off
- The M1 is closed southbound at J8, near Hemel Hempstead, because of a serious collision
- A lorry has jack-knifed on the A1 in the north east, with the road closed southbound near junction 52, between Scotch Corner and Catterick
- A lorry has also come off the road on the A90 at Stonehaven in Aberdeenshire
- The AA says it has received three times as many breakdown call-outs than normal on Wednesday morning
- A number of other roads across the UK have been affected so check your local BBC website for updates
Other disruption includes:
- Chatham Ski and Snowboard Centre in Gillingham, Kent, has been forced to close because of the heavy snow
- The University of Glasgow and Glasgow School of Art are shut due to the weather
- The Scottish Premiership has postponed its games for Wednesday night and Doncaster Racecourse has cancelled its meeting on Friday
- Bin collections have been suspended across Essex
The Met Office has issued an amber warning – meaning widespread damage, travel and power disruption is likely – for parts of Scotland and the north of England.
It has predicted up to 10cm (4in) of snow between 10:15 GMT on Wednesday and 18:00 on Thursday – although it could be as much as 40cm (15in) over hills.
The East Midlands, the east and Yorkshire and Humber are also subject to an amber warning, with up to 15cm (6 inches) of snow predicted to fall between 10:40 and 16:00 on Wednesday.
Another amber warning is in place across the south of England, Wales and the West Midlands between 12:00 on Thursday and 08:00 Friday, predicting up to 20cm of snow – reaching 50cm over parts of Dartmoor and Exmoor.
A less severe yellow warning for snow is in place for parts of Scotland, northern England, Northern Ireland, the south-west and south-east England, the West Midlands, East Midlands and London until 23:55 GMT.
The UK is experiencing what is predicted to be the coldest week of the winter in five years.
The lowest temperature recorded overnight was -11.7C in South Farnborough, Hampshire.
Other particularly cold spots were Cairnwell, in Scotland, Ravensworth, in North Yorkshire, and Frittenden in Kent.
On Tuesday three people died following a crash in Lincolnshire while one man died in a crash in Cambridgeshire, amid warnings of treacherous conditions on roads.
In the early hours of Wednesday, a crash between a car and motorcycle on the M3 in Surrey left one person seriously injured.
From Thursday, the Siberian system is set to meet with Storm Emma, with forecasters predicting more snow, gales and sleet.