Britain is bracing itself for heavy snow after the Met Office issued amber weather warnings – indicating a possible risk to life – for large swathes of the country.
The alert has been raised for snow showers on Tuesday morning, covering the South East, northeastern England and the East Midlands.
Forecasters predict up to 15cm (6in) of snow could fall in just a few hours with the warnings in place between 2am and 12pm.
Drivers, commuters and those looking to catch planes are being warned they could face even more severe disruption on Tuesday morning as the freezing temperatures continue into the week.
An amber warning is also in place for the northeast of England and Scotland between 6am on Wednesday and 12pm on Thursday, with up to 40cm (16in) of snow expected to fall during that period.
There could be blizzards, gales and sleet on Thursday as Storm Emma hits the cold air already over the UK.
The wind chill, which could see parts of the country drop as low as -15C (23F), is on par with forecasts for parts of northern Norway and Iceland.
A less severe yellow warning is in place until Friday across many other areas of the UK.
Hundreds of trains and dozens of flights have already been cancelled as a result of the harsh conditions.
Met Office forecaster Marco Petagna said people should "try to avoid" travelling where and when the amber warnings apply.
Schools could also be hit by closures due to heavy snowfall this week.
Paul Whiteman, general secretary of the National Association of Head Teachers (NAHT), said: "Schools should stay open if they possibly can but safety concerns are the most important.
"If it's not safe for the school to be open then it is fine to close."
The Met Office said rural communities could be cut off, with power outages and disruption to mobile phone services likely.
More than 200 Greater Anglia trains are no longer running on Monday and Tuesday, while South Eastern passengers were told to finish journeys by 6pm on Monday to avoid disruption.
Passengers were also warned to expected delays and cancellations on c2c, London Overground, South Western Railway, Southeastern, Stansted Express and TfL Rail.
Train operator Great Northern said it would run a "heavily reduced timetable" on Tuesday.
British Airways said it had cancelled over 68 short-haul flights.
Public Health England has urged people to check on elderly or vulnerable neighbours during the cold spell.
Members of the public have also been asked to look out for the homeless and report anyone sleeping rough in the freezing conditions to their local council.
Sky News weather forecaster Chris England said the snowfalls are set to become more severe this week.
He said: "Snow showers will continue over many eastern areas tonight, turning heavy in places, with some getting across to northwest England later.
"Parts of eastern England in particular can expect around 15cm (6in) of snow, leading to some disruption. It will be freezing everywhere.
"Tuesday morning will see snow showers developing more widely across Britain, bringing a risk of some disruption, while rain, sleet and snow showers will push into Northern Ireland and the east of Ireland. The west of Ireland will be mostly fine.
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"Britain will be near freezing all day, with a bitterly cold wind. Wednesday will bring widespread snow showers, some heavy. It will be very cold, with many places not getting above freezing all day.
"Thursday will bring further heavy snow showers and the risk of more general snow in the South."