Heavy snow has fallen across large parts of England, disrupting travel and closing hundreds 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 runways.
Road and rail travel was also badly hit by up to 6in (15cm) of snow.
The Met Office has issued an extreme weather warning for England, Wales and parts of eastern Scotland. More snow is forecast later and on Tuesday.
'Dangerous driving'
Thousands of school children across England woke up to the news their school was closed for the day.
In Essex more than 450 are shut and 255 Berkshire schools are now closed. Leicestershire and Rutland gave pupils at 200 schools the day off.
A snowman greeted BBC website reader David Shipway on his doorstep
Hundreds more are closed across 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.
Air travel has been badly affected.
London City, Luton and Southampton airports are closed. Norwich and Leeds Bradford airports have now reopened.
Heathrow Airport has had to shut both runways and says it will take about two hours to reopen.
BAA has warned passengers travelling to and from Heathrow and Stansted airports to expect "significant delays and cancellations".
BA says it has cancelled 65 flights, while 73 Ryanair flights are grounded.
Gatwick is open, although fewer aircraft are able to take off. Some 23 flights had to be cancelled and 18 diverted to other airports after Gatwick closed its runway for two hours on Sunday evening.
There is no service on the Gatwick Express.
IMPACT OF THE SNOWFALL
Hundreds of schools closed
London buses withdrawn
Both runways closed at Heathrow
London City and Luton airports closed
Passengers stranded at Gatwick
Train services cancelled
Transport for London
Passengers due to fly out during the day are being urged to double-check their flights are not affected before they begin their journeys.
On the roads, motorists were being warned of dangerous driving conditions.
A string of accidents on the M25, A12 and A14 caused problems for motorists.
The M25 is closed clockwise between Reigate and Leatherhead, junctions 8 and 9 and there are reports of several vehicles becoming stuck in the snow.
The A66 in the north-west of England, between Brough and Stainmore Summit, is expected to stay closed all day.
Two lanes of the M62 over the Pennines were closed for several hours last night, but have now re-opened.
There are warnings on the M6 in Greater Manchester, while the M60, M66 and M602 have also been affected by the snow.
The London-bound M20 in Kent was temporarily closed between junction nine at the Ashford turn-off and junction eight at the Leeds Castle turn-off.
And there was more bad news for people trying to use public transport.
All London buses have been withdrawn from service due "to adverse weather and dangerous driving conditions"
A BBC reporter said he saw many people left confused and distressed after waiting for long periods for buses, only to see them drive past without being in service.
The snow delighted many but bought travel misery for others
There are also problems on the London Underground and rail network.
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.
Scotland Yard said it had reports of a number of crashes in the early hours, though no-one was seriously hurt.
'Flirting with hypothermia'
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.
BBC website reader Fiona Morse snapped Surrey Quays under snow
BBC weather presenter Jay Wynne said further snow showers were expected during the morning across much of England, eastern parts of Wales and eastern parts of Scotland.
Temperatures in the afternoon would not reach above freezing, he said.
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, blizzard conditions".
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.
It added that the number of traffic officer patrols had been increased, there were extra staff to control rooms across the country and gritters had been working through the night.
The AA has warned motorists to ensure they keep warm clothes in their vehicles if they were planning to go out, or else risk "flirting with hypothermia".