Heavy snow has brought Britain to a virtual standstill with bus services suspended, trains cancelled and flights delayed. Much of the UK is blanketed in snow after the heaviest falls in several years and the Met Office issued an extreme weather warning for London and the south east of England with heavy and drifting snow making driving conditions treacherous. Both runways at Heathrow Airport in west London are snowbound forcing the cancellation of hundreds of flights. London City Airport has also been closed. London Transport suspended its bus services because of dangerous conditions on the roads. A number of train services linking London and the south coast were also delayed or cancelled as snow drifted onto the tracks. Gatwick Airport and London City Airport were both temporarily closed as their runways were de-iced. Transport for London said in a message on its website: "All London buses have been withdrawn from service due to adverse weather and dangerous road conditions." Parts of the motorway network in the south east have ground to a halt. Scotland Yard said it had received reports of a number of crashes in the early hours although no one was seriously hurt. The AA has warned motorists to ensure they keep warm clothes in the car after dealing with unprepared drivers "flirting with hypothermia" at breakdowns during the last big freeze. AA Patrol of the Year Andy Taylor said: "People often treat their car as an overcoat. But when you break down you are suddenly vulnerable to the weather. "If you break down on a motorway, the safety advice is to get out of the car and wait behind the barrier. Unless you have extra clothes you really are flirting with hypothermia." BAA warned passengers travelling to and from Heathrow and Stansted airports to expect "significant delays and cancellations". A spokesman said one runway at Heathrow was closed and urged passengers to check for the latest information before leaving for the airport. He said: "With bus and rail services affected and difficult road conditions across the country, please take extra care and time if you do intend travelling to the airport." Meanwhile, Southeastern said it had cancelled all rail services into London after heavy snow fell overnight Kent and Sussex. A spokeswoman said: "We are endeavouring to run coastal services in Kent and East Sussex but all trains to London are cancelled." Many schools in the south east were forced to close due to the weather. Many businesses were also expected to close down for the day as workers struggle to make journeys in on icy roads.