Millions of Britons faced travel misery today with planes grounded, rail services cancelled and roads rendered impassable on what is traditionally the busiest weekend before Christmas.
Plunging temperatures and heavy snow saw large swathes of the country grind to a standstill, as London's Gatwick Airport closed its runway and British Airways cancelled flights at Heathrow.
Overnight blizzards and plummeting temperatures buckled a huge chunk of the nation's road and rail networks, as the latest blast of wintery weather showed little sign of easing.
This weekend is expected to be the busiest for retailers and commuters in the run-up to December 25 but many are expected to face chaos as the transport network bears the brunt of the big freeze.
Air passengers have already faced disruption at Exeter, London City, Aberdeen and Cardiff airports, while Belfast City was closed last night, with flights expected to resume at 11am.
Southampton Airport was closed until 12pm and flights to certain destinations from Birmingham were also grounded.
Delays and cancellations were expected at Belfast International while a spokesman for Heathrow said the airport was open but would be "challenged" today.
"Heathrow is fully operational but we are expecting more snow and planning for the worst," he said.
"BA passengers must stay at home as all BA flights are cancelled. At this time there are no significant delays at other terminals.
"All passengers must check with airlines before setting off as the situation will worsen."
A Gatwick airport said efforts were now under way to clear the snow.
"We've had heavy snowfall solidly now for the past hour and it continues to fall," a spokesman said.
"We're now out there ploughing, sweeping and brushing the snow away and will continue to do that until it's safe to resume operations."
Officials advised all passengers to check their flight status before travel.
There was also widespread chaos on major roads and motorways.
Hundreds of motorists were left stranded on the M6 in Greater Manchester as the North West of England was hit with reported deluges of up to 25cm of snow.
Meanwhile, parts of the south were also blanketed and as much as a foot of the white stuff is expected to fall on parts of the South East and the Midlands today.
Forecasters expect the mercury will struggle to break freezing in the next 24 hours, after overnight temperatures dropped as low as minus 13C in Chesham, Buckinghamshire, with Gatwick producing a reading of minus 11C.
A fleet of 47 snow ploughs, tractors and other vehicles were deployed in a frantic effort to clear the snow from the runway at Gatwick as it continued to fall.
Some 150 staff were drafted in to carry out the operation in an attempt to clear the ground and create safe conditions for take-offs.
But as soon as a patch was cleared, it was almost instantly covered again by another coating.
Regulations require the airport to suspend flights if snow lies thicker than 3cm on the ground.
And the heavy snowfall was expected to continue for at least another hour.
Extra ground staff were working in the terminal to deal with frustrated and disappointed passengers facing long waits.
Free internet access was provided to help would-be travellers book alternative flights and attempt to rescue their holiday plans.
The airport spokesman said: "When you get a situation like this the welfare of the passengers is of the utmost priority.
"We've told them to expect delays and cancellations throughout."
Almost 600 flights were scheduled to operate from Gatwick today, with at least 138 cancelled by mid-morning.
British Airways grounded all its flights out of Heathrow until at least 5pm this afternoon, despite the airport's operator saying it is "fully operational".
A spokeswoman from the airline said: "The weather at Heathrow now is quite appalling."
"We knew that severe weather conditions were expected, so rather than asking passengers to travel down from Scotland, for example, just to have their flight cancelled, we think it's better to tell all our customers that flights are cancelled.
"We need to give our customers some certainty."
She said the airline regretted inconveniencing passengers, but insisted it was the correct decision.
"This costs British Airways a huge amount of money, so this is not a decision we take lightly," she said.
"There's a backlog from yesterday and that has an impact on today's customers.
"The main thing is we don't want customers turning up today to get stranded and not be able to get out by car or by plane.
She said the airline is providing people with food and drinks, and putting people up in rooms.
Warnings were also issued to communities in Hampshire and the Isle of Wight, with snow depths reaching up to 12cm in some areas.
Temperatures on the Isle of Wight are due to plunge to -10 in rural parts, presenting black ice hazards to drivers.
Christmas shoppers were told to stick to main roads, which have been gritted as a priority along with trunk roads, motorways and town and city centres.
Festive party-goers out in the towns and cities this weekend were encouraged to put on warm clothing as the mercury drops overnight.
Salting lorries from Hampshire County Council have been running round-the-clock since 6pm last night on the priority one routes.
More than 1,000 salt bins have been placed in communities to enable residents to help themselves to keep pavements and roads safe.
All major and most minor roads in the two counties were passable with care but there are fears that slush will freeze later, Hampshire Police said.
In the North West, one of the worst affected regions, police and highways officers were providing food and water to stranded drivers.
Commuter Stephen Derbyshire said he was stuck for five hours on the M6 in Greater Manchester, which has now reopened. He was forced to complete the last two hours of his journey on foot.
"It is absolute mayhem," he told BBC Radio Five Live.
Drivers were advised to avoid parts of the M6 and M61 in Lancashire following a number of road traffic collisions due to snow and ice. The M58 and M57 were also affected by the weather.
Yesterday, nearly a quarter of all train services experienced delays and a similar picture was expected today with operators in the south, including Southeastern, South West Trains and Southern Railway, running reduced services.
Wintry conditions were also causing delays in Scotland but new transport minister Keith Brown said a multi-agency response was under way to keep the north east and Highlands and Islands moving in the face of severe winter weather.
The country's major airports were open but experiencing delays and cancellations because of the knock-on effect on heavy snowfall in the south-east of England.
But the cold weather was good news for some. Many of Scotland's mountain sports resorts were reporting good conditions to take to the slopes.
Severe weather warnings of heavy snow and widespread icy roads are in place in London and the South East, the South West, the Midlands, the North West and Yorkshire and Humber. Wales, northern Scotland and Northern Ireland - which had experienced its heaviest snow for 25 years - were also issued with severe warnings.
Mark Seltzer, a forecaster for the Met Office, predicted this could be the coldest December on record, with a current average temperature of minus 0.7C - five degrees below the long-term average.
"A lot of snow has fallen across the north-west of England overnight but it should ease off during the day," he added.
"A lot of snow also fell in the South West overnight and it will progressively move into the South East and parts of the Midlands and East Anglia throughout the day. There could be local accumulations of 20-25cm.
"Temperatures will struggle to get over freezing and although the snow should ease off tonight, it will return to eastern areas on Sunday."
Yesterday, a man injured in a car crash near Eastbourne, in East Sussex, died after the ambulance he was being taken to hospital in left a country road in poor weather conditions and careered into a bush.
Elsewhere, two pedestrians died after being hit by a Range Rover in Glasgow; a 17-year-old boy was killed when his car ploughed off the road in Bilsington, Kent; a woman in her 60s, thought to be from the Inverness area, was killed in a three-vehicle pile-up in Huntly, Aberdeenshire; and a 19-year-old man was stabbed in the neck in Sheffield after asking a group of teenagers to stop throwing snowballs.
Thames Water reported a "near four-fold" increase in burst pipes caused by the sub-zero temperatures.
Transport Secretary Philip Hammond yesterday met RAC Foundation chairman David Quarmby to receive his audit on how well Britain was prepared for poor weather conditions. The report is due to be published next week.
Transport minister Norman Baker has written to local authorities reminding them of earlier independent recommendations to reduce the amount of salt they spread on roads in an attempt to conserve stocks.
Disgruntled passengers stuck at airport terminals tweeted their irritation on the microblogging site Twitter.
Corine Wood, waiting at Gatwick Airport, wrote: "Our north terminal bound shuttle is stuck in the snow. SAVE US!"
A passenger at Heathrow going by the name of Fridalust asked: "How come British Airways flights have been cancelled but 'most other flights' are ok!? What's the difference between them?"
And Lara Brill tweeted: "They always seem very surprised and unprepared when it snows, which is foolish."
Gatwick Airport said the runway would be shut until at least 3pm.
The AA had attended around 4,000 breakdowns across the UK by 10.30am, with calls peaking at 1,200 every hour - around twice the normal rate. By the end of the day, it expects to attend around 18,000 call-outs, compared to around 9,000 on an average Saturday.
A spokesman said the southern half of the country was the worst affected, with drivers suffering most problems within the M25 and in the Home Counties. Call-outs were also high in Somerset and Newcastle.
Extra AA patrols and 'storm-chasing' Land Rover crews were working across the country to help motorists in trouble.
Darron Burness, head of AA special operations, said driving conditions were "extremely difficult" in some regions.
"One of the biggest problems is that large amounts of snow are falling very quickly on to frozen surfaces, making driving hazardous," he added.
"Around the western side of the M25, three to four inches of snow has fallen in two hours.
"Some may have no choice but to travel, although with road conditions unlikely to improve much over the weekend, drivers should exercise a lot of caution and be prepared for some long delays in places."
Dozens of football fixtures fell victim to the weather, as snow blanketed pitches and the icy conditions made getting to grounds too treacherous for fans.
In the North West, Wigan's clash against Aston Villa at the DW Stadium, as well as Liverpool's match versus Fulham at Anfield were cancelled.
Cardiff's home tie against Burnley and Portsmouth's game at Fratton Park with Scunthorpe were called off in the Championship.
In Scotland, Premier League games between Rangers and St Mirren, Aberdeen and Motherwell, St Johnstone and Dundee Utd, and Hamilton and Celtic were cancelled.
Over 20 more lower league matches were cancelled in England, as well as around eight non-league clashes. About a dozen lower league games in Scotland were also called off.
Racing meets around the country were also forced to be abandoned, as well as many other sporting fixtures.
Birmingham's Premier League fixture at home to Newcastle was also postponed due to snow and freezing conditions, the Midlands club confirmed.
Birmingham Airport suspended all of its flights until at least 1pm while it clears snow from the runway.
A spokesman warned that flights expected to leave the airport this afternoon may be cancelled or delayed because of the adverse conditions.
West Midlands Ambulance Service said yesterday was its busiest day since regional records began, even beating the last three consecutive New Years Eve/Days.
A spokeswoman said it provided 3,941 responses throughout the West Midlands region, approximately 120 more responses compared to last year's New Years Eve/Day, which is traditionally the busiest time for the ambulance service.
A West Midlands Ambulance Service spokeswoman said: "A large number of these calls were to reports of breathing problems, road traffic collisions and slips, trips and falls, which have come about due to the sudden onset of cold weather.
"With the wintry weather set to stay over the next couple of days, the public is being urged to take extra precautions to try to avoid any unnecessary accidents."
Up to 15cm of snow is expected to hit London today while weathermen suggested around 20cm could fall in parts of North Wales, the Midlands and south east England.
Lindsay Dovey, a forecaster with MeteoGroup, the weather division of the Press Association, said temperatures dropped to minus 15.5C in Aviemore, Scotland, this morning.
"There has been a band of snow that is stretching from North Wales right across to the far south east of England, pivoting northwards over London," she said.
"It is going to be around for a while, probably for most of the day, but will hopefully die out tonight when conditions are going to improve."
The mercury could plunge to minus 20C in parts of Scotland, she said.