Heavy snow has forced hundreds of schools to close and caused disruption to travel.
The icy blast overnight has caused misery for morning commuters with scores of trains cancelled.
Many schools across the country shut on Tuesday, including more than 200 in Wales, 131 in Kent and 62 in East Sussex.
Some roads were closed after a blanketing of snow, with police forces reporting treacherous driving conditions and blocked routes.
The Met Office said several centimetres of snow had fallen in some parts overnight – with between 3cm and 4cm in Newcastle and Northumberland.
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Farnborough in Hampshire saw the lowest overnight temperature of minus 8.9C.
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 dozens flights from Heathrow Airport, while easyJet warned disruption to its flights was expected.
Cars covered in snow in Ashford, Kent (Gareth Fuller/PA)
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: “There will also be further disruption throughout the day as these showers become widespread across the country.”
Conditions are not likely to improve for several days, with forecasters warning that snow will continue well into the week.
Retailers said they have not seen widespread panic buying, after some shoppers reported supermarkets were busier than usual.
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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.