The highest possible extreme warning for parts of south Wales and the south west of England urges residents to "take action now to keep yourself and others safe" as Storm Emma, rolling in from the Atlantic, looks poised to collide with the "Beast from the East'" weather front.
As another day of extreme weather left hundreds of drivers stranded overnight and more roads impassable in up to a foot of snow in Scotland and the north of England, further widespread accumulations are expected to bring up to 40cm (15in) in some areas.
Although a red alert for central Scotland remains in place until 10am on Thursday, police said everyone barring emergency workers should avoid all travel until weather conditions improve. Hundreds of schools remain closed across Britain.
The second red alert - which stretches from north of Cardiff to south of Exeter - is for 3pm on Thursday to 2am on Friday.
The Met Office said: "A spell of heavy snow and blizzard conditions is expected. The snow will be accompanied by very strong easterly winds, leading to severe drifting.
"Some places could also see significant ice build up due to freezing rain from later on Thursday evening, most likely in the south of the area. Long delays and cancellations of public transport seem highly likely.
"Some roads are likely to become blocked by deep snow, stranding vehicles and passengers. Long interruptions to power supplies and other utilities are very likely to occur, along with damage to trees and other structures due to heavy snow or ice."
<br> In Lincolnshire, police earlier warned that most roads had become impassable, with the force receiving reports of some remote villages being "totally cut off under 2ft of snow".
One major road, the A52, was closed between Boston and Skegness all night, with not even a snowplough able to get through.
Motorists on the M80 near Glasgow reported being stuck for up to 13 hours, with some spending the night in their cars and others abandoning their vehicles on the motorway.
As well as the red alerts, amber warnings have also been issued for the north-east of England and the central belt of Scotland, in place between 10am and 8pm on Thursday, and for south west England and south Wales from 12pm until 8am on Friday.
On top of these, yellow snow warnings have been issued for vast swathes of southern, central, and northern England, Northern Ireland and Wales - in place for most of Thursday.
The prolonged cold spell will collide with Storm Emma, which is hitting southern and western parts of the UK. Blizzards, strong winds and travel disruption are predicted for southern, western and central England, as well as parts of Wales and Northern Ireland.
Temperatures plunged to a widespread low of between -6C (21F) and -4C (25F) overnight, and forecasters have warned it could feel as cold as -11C (12.2F) during the day as the winds continue to strengthen. Gusts of up to 60mph could also bring "blizzard-like conditions".
Met Office forecaster Simon Partridge said some parts of the south west could see up to 50cm (15in) of snow by Friday morning.
"If you don't have to go anywhere over the next few days, stay at home," he warned.
Disruption across the country's road and rail network as well as at airports is expected to ensue throughout Thursday.
Severe weather has caused another day of travel disruption on the roads, to flights and to trains. Travellers were urged to check the latest situation before setting off on their journeys. Here are some of the latest reported issues:
Lincolnshire Police warned overnight that the A52 in Lincolnshire had become "impassable" between Boston and Skegness due to drifting snow, and warned that drivers faced becoming stranded.
"We are not in a position to recover you", the force tweeted.
The fire and rescue service added that even a snowplough was unable to get through to clear the route.
Several other roads in Lincolnshire have also been shut, including the A17 and the A15 between the M180 and Lincoln.
The A66 in Cumbria was closed in both directions between Scotch Corner and Brough, due to "severe snowfall", while the A56 in Lancashire was likely to be closed for "several hours" due to a lorry crash between the M65 and A680.
Northumbria Police said the A68 was closed between Corbridge and Ridsdale.
In Derbyshire, the Woodhead Pass was shut in both directions between Tintwistle and Flouch.
In Scotland, motorists reported being stuck on the M80 all night as heavy snow continued. The motorway has been closed southbound between junctions 8 and 9.
Police Scotland warned people not to travel unless they were an emergency worker.
Nearly all train operators across the UK are warning of cancellations and disruption on Thursday.
Virgin Trains East Coast warned that anyone travelling north of Newcastle should defer their journey, and that there would be a number of alterations to its services.
TfL Rail also cancelled or amended some services.
Northern said the severe weather had left several routes blocked, with delays and cancellations across the whole network.
East Midlands Trains also reported several alterations and cancellations.
Several airports reported disruption due to the weather, with Gatwick Airport expecting "a large number of cancellations and delays to flights".
Glasgow Airport said it will be closed until 3pm on Thursday following heavy snow showers overnight.
Edinburgh airport said it was currently closed because weather conditions meant it and the airlines operating there "do not have the critical mass of staff needed to run operations securely".
Heathrow said it had "consolidated" its flight schedule "to provide more certainty around departing flights, and ensure the safety of our passengers and colleagues", while Gatwick said "many flights are cancelled or delayed today".
Gatwick Airport said it was expecting a "large number" of cancellations and delays, while Heathrow urged passengers to check with their airline before travelling.
Manchester Airport said a total of 30 flights had been cancelled.
The airport said: "Due to the continued bad weather we are seeing 30 cancellations so far today. We'll update on these as soon as possible and provide a full list. In the meantime passengers can check our website and with their airline."
The runway at East Midlands Airport was temporarily closed on Thursday morning, and travellers were warned it could affect flights.
Conditions to worsen as ice set to form on snow
The extreme weather shows no sign of letting up, with fears snow in the south west could turn into freezing rain on Friday - making conditions "even more treacherous".
Heading into Friday the snow in the south west might turn into freezing rain and could give a glaze of ice on top any lying snow - making conditions even more treacherous.
The Met Office said the highest temperature in the UK on Wednesday was 1.7C (35F) recorded in Katesbridge, County Down and the coldest in Braemar, Aberdeenshire at -5.3C (22F).
Mr Snell warned: "Generally many places on Thursday will struggle to get anywhere near -1C, a few people will come up above, but many of us will be firmly below freezing."
What the different weather warnings mean
As icy blasts gripped Britain on Wednesday, a man in his 60s died after being pulled from a lake in south-east London.
The victim fell into Danson Park, in Welling, and was rescued by a member of the public, London Fire Brigade said.
A Metropolitan Police spokesman said the man was later pronounced dead in hospital.
Elsewhere, a young boy had a lucky escape when he was rescued from a frozen lake.
Firefighters rescued the youngster, who tried to run to the middle of a frozen lake and got trapped when the ice broke up at Laindon near Basildon, Essex on Wednesday afternoon
He was said to have been cold but otherwise unhurt.
Dozens die in freezing European weather
Fresh heavy snowfalls and icy blizzards are also expected to continue lashing large swatches of Europe as the continent shivers in a deadly deep-freeze that has gripped countries from the far north to the Mediterranean south.
Schools are shut and weather agencies predict the brutal cold will continue as the death toll from the freezing snap rose to around 48 since last Friday, with icy conditions causing accidents and endangering vulnerable rough sleepers.
The victims also include 18 people killed in Poland, six in the Czech Republic, five in Lithuania, four each in France and Slovakia, two each in Italy, Serbia, Romania and Slovenia and one in Spain.