People are being warned to avoid non-essential travel
Heavy snow has fallen across large parts of the UK, disrupting travel and closing thousands of schools.
South-east England has the worst snow it has seen for 18 years, causing all London buses to be pulled from service and the closure of Heathrow's runways.
The Met Office has issued an extreme weather warning for England, Wales and parts of eastern Scotland.
By late Monday, the South East could be under a foot (30cm) of snow and the North East under 20 inches (50cm).
Speaking at a press conference in Downing Street, Prime Minister Gordon Brown said: "We are doing everything in our power to ensure services, road, rail and airports are open as quickly as possible, and we are continuing to monitor this throughout the day."
BBC weather forecaster Tomasz Schafernaker said the worst-affected area has been the Thames Valley and Greater London where the transport network is at its most concentrated, which is why there has been such chaos.
A snowman greeted BBC website reader David Shipway on his doorstep
He said some parts of London have seen up to 8 inches (20 cm) of snow and through Monday evening there will be another band of snow.
Overall, some parts of London could see a foot (30cm) of snow, while northern England and the Pennines could see as much as 20 inches (50cm) - made worse by strong winds which will cause snow to drift, he added.
Thousands of school children across England and Wales woke up to the news that their school was closed for the day.
Most Surrey schools are closed, while in Essex, more than 450 shut and 255 Berkshire schools are closed. Leicestershire and Rutland gave pupils at 200 schools the day off.
Thousands more are closed across Wales, London, north-east England, East and West Sussex, Kent, Norfolk Cambridgeshire, Lincolnshire and Suffolk, where snow-ploughs were out on the county's roads for the first time in more than 15 years.
Hospitals in London have put out an emergency call to staff to come into work because of a big increase in 999 calls, while many non-urgent operations have been cancelled and ambulance response times have slowed.
Bad weather has also forced the closure of the Old Bailey courts in London and crown courts in Southwark, Blackfriars and Lewes.
And there was more bad news for people trying to use public transport in the South East of England.
All London buses have been withdrawn from service due "to adverse weather and dangerous driving conditions".
Many Tube lines are partially suspended and the Circle and Hammersmith and City lines entirely suspended.
South West Trains is running a reduced service on most routes and passengers are advised not to travel unless their journey is essential.
David Brown of Transport for London said the situation was "exceptional".
He added: "We haven't had a circumstance like this for over two decades. We were prepared in the sense that all our cold weather plans on the underground were put into place.
"But I think that actually the volume of the snow falling during the middle of the night was very difficult for us."
The Highways Agency said there have been too many minor accidents on the roads "to put a number on".
The agency recommended people should only make essential journeys.
Eurostar services are currently operating between the UK and the continent, but may be delayed due to heavy snow in the UK and northern France.
In the late afternoon, more heavy snow is forecast to hit the eastern part of England, including East Anglia, London and the Home Counties, accompanied by "strong winds [and] blizzard conditions".
BBC website reader Fiona Morse snapped snowy Surrey Quays, London
This will work its way up through the Midlands, northern England and eastern Scotland.
Evening temperatures will fall to about -1C for many areas of England and eastern Wales, with the weather turning icy by Tuesday morning making road travel treacherous.
There will be minor snowfalls and temperatures will reach about 4C in most areas by Tuesday afternoon.
One motorist, driving in the Midlands on Monday, told the BBC conditions were much clearer there, but highway officers told him they had responded to 1,735 incidents over a 24-hour period in the East Midlands alone.
The Highways Agency said the organisation was well prepared to deal with snowfall over roads after criticism over its reaction to severe weather in 2003.
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