Heavy snow has fallen across large parts of England disrupting travel and closing schools as forecasters warned of the most severe snow for six years.
South-east England was hardest hit, with London buses pulled from service and airport runways closing.
The snow also resulted in jack-knifed lorries blocking the M1 and the M25.
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.
MeteoGroup, the Press Association's weather division, said up to 10cm of snow had already fallen in some parts of Greater London, with 13cm reported near Stansted and 6cm at Heathrow Airport.
A spokeswoman said: "The South West has escaped the worst of the snow, but we could see another 10cm in Greater London over the next 24 hours, and even more than that in Yorkshire and the Pennines."
Air travel has been badly affected.
London City airport is closed, and a runway at Southampton airport has also shut.
Heathrow Airport has had to shut one of its two 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". Leeds Bradford Airport is also experiencing long delays.
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
London buses withdrawn
Gatwick Airport temporarily closed
Passengers stranded at Gatwick
Train services cancelled
Jack-knifed lorries obstruct M1 and M25
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.
There are warnings on the M6 in Greater Manchester, while the M60, M66 and M602 have also been affected by the snow.
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 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All London buses have been withdrawn from service due "to adverse weather and dangerous driving conditions"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 Highways Agency said people should only take essential journeys
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.
One motorist, driving in the Midlands, 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".
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.