Heavy snowfall and freezing temperatures are causing travel disruption on Tuesday morning as the wintry weather looks set to continue throughout the early rush hour.
Roads across the UK have already seen a blanketing of snow, with police forces reporting treacherous driving conditions and blocked routes.
Highways England said the M20 in Kent had become blocked by a "number" of stranded lorries overnight, while the A249 was shut due to a crash.
Areas around Harrogate and Craven were also particularly treacherous, according to North Yorkshire Police.
The Met Office said several centimetres of snow had fallen in some parts over the night - with 1cm in Kent by 5am, and between 3cm and 4cm in Newcastle and Northumberland.
Heavy showers were expected to continue throughout the morning and cripple much of the country's travel network.
Trains have also been affected by the snowfall, with cancellations and disruptions on lines across the country.
Southeastern, which operates in Kent, has cancelled dozens of trains, including several to London St Pancras, London Victoria and Cannon Street, while other companies affected include Southern, Greater Anglia and Great Northern.
British Airways has cancelled several flights from Heathrow Airport, while easyJet warned disruption to its flights was expected. School closures were also likely.
Amber warnings for snow are in place for the South East and North East of England and the East Midlands until midday, while a yellow warning covering much of the country is in force until midnight.
Forecasters are predicting 5cm-10cm of snow will fall for most areas of the UK, with up to 40cm possible for higher grounds in Scotland.
Met Office meteorologist Emma Sillitoe said: "During the rush hour there will still be some really heavy showers around, making their way towards London.
"There will also be further disruption throughout the day as these showers become widespread across the country."
Temperatures plummeted to minus 5C in the capital overnight, and the mercury will struggle to get much above freezing during the day.
Conditions are not likely to improve for several days, with forecasters warning that snow will continue well into the week.
An amber snow warning is in place for the North East of England and Scotland from 6am on Wednesday to 12pm on Thursday, with up to 40cm of snow expected to fall during that period.
Forecaster Frank Saunders said parts of the country could see their "coldest spell of weather since at least 2013, and possibly since 1991".
It is expected that the mercury could plummet to minus 15C by midweek where there is snow on the ground, rivalling temperatures forecast for parts of northern Norway and Iceland.
From Thursday, forecasters predict that another weather system, Storm Emma, will bring blizzards, gales and sleet as it meets the chilly "Beast from the East" later this week.
The storm, named by the Portuguese Met Service, will move north through Europe and is due to hit the UK on Thursday and Friday, and will be "significantly disruptive", bringing the risk of power cuts and transport delays.