Snow has hit parts of England with up to 4in (10cm) expected to fall in some areas, prompting fears of travel chaos.
The first wave of snow dusted eastern England, with a second heavier band expected to move south and east across the country later.
The Met Office has upgraded its "cold weather action" warning to level three - one underneath a national emergency - for all of England, urging social and healthcare services to target specific actions at high-risk groups.
It said: "This weather could increase the health risks to vulnerable patients and disrupt the delivery of services."
A number of yellow and amber warnings are also in place across the country to highlight the hazardous conditions and likelihood of disruption to travel services caused by the ice, snow and freezing temperatures.
The heaviest snow of up to 4in (10cm) will fall in Lincolnshire, Yorkshire and East Anglia, while central England will widely see up to 2in (5cm) with temperatures hovering around 1C (33.8F).
Matt Dobson, senior forecaster for MeteoGroup, the weather division of the Press Association, said: "We have two bands of snow to worry about today. The first is over south east England and East Anglia this morning but it is coming down quite slowly and it's light.
"There is another band of rain, sleet and snow moving out of eastern Scotland across north-west England and Wales and moving south-east all the time.
"If you draw a line from Manchester to London, south and west of the line there will be mainly rain and sleet. North and east of that line it will fall mainly as sleet and snow. Yorkshire, Lincolnshire and East Anglia, they are going to get the worst."
Temperatures are expected to rise to up to 8C (46F) in the west of England and Wales but struggle to rise above zero in the rest of the country. The band of snow is expected to stall overnight, bringing a risk of further falls in Norfolk and Suffolk, but the rest of the country should see a dry Tuesday, Mr Dobson added.