Widespread travel chaos caused by some of the severest winter weather for a century left Christmas commuter plans in tatters and swathes of the country paralysed today.
Delays and cancellations at a host of UK airports continued to hit thousands of passengers, while plummeting overnight temperatures left almost the entire road network caked in treacherous black ice.
After shutting yesterday, Heathrow said it would not be letting any flights land on its runway, with only a "handful" of departures taking place, pouring misery on hundreds of stranded people who were forced to spend the night in terminals.
Gatwick Airport reopened after planes were grounded for large period of yesterday, but widespread disruption was expected with officials advising passengers to check with their airline before travelling.
Stansted, Luton, Exeter, London City, Aberdeen, Birmingham, Bristol, Southampton, Cardiff and Birmingham airports also said flights would be subject to delays and cancellations.
A spokeswoman for Heathrow, which experienced temperatures of minus 5.2C overnight, said: "Heathrow Airport will not be accepting arrivals on Sunday, and will only manage a handful of departures as our airfield team continues to deal with the impacts of yesterday's bad weather and prepares the airport for a full reopening on Monday.
"We are extremely sorry for the disruption this will cause to passengers and airlines and we stress that passengers must check with their airlines before travelling to the airport. We will provide regular updates.
"We have listened carefully to the advice of our airside operations team and have reluctantly judged that while Heathrow's northern runway remains clear, the change in temperature overnight has led to a significant build-up of ice on parking stands around the planes and this requires the airfield to remain closed until it is safe to move planes around."
The airport said several hundred people were working to treat airside areas and keep passengers in the terminal as warm and comfortable as possible.
"Safety is our first priority and we hope this course of action will allow us to offer a fuller service to passengers and airlines using Heathrow tomorrow," added the spokeswoman.
Hundreds of thousands of Britons had been due to fly this weekend, with four million expected to go abroad, according to travel association Abta.
Singer Lily Allen was one of the travellers caught up in the chaos after boarding and then being forced to disembark from a plane at Heathrow yesterday.
US student Zack Robinson told BBC Radio 5 Live he sat on a plane for eight hours waiting to take off before being told it was cancelled.
Forecasters said the UK was hit by abnormally low temperatures overnight, with most parts of the country struggling to get over minus 5C.
The mercury dropped to minus 19C in Pershore in the Midlands, and minus 14C in Chesham, Buckinghamshire, while fresh snow fell in eastern England and Scotland.
Snow was today expected in northern and eastern Scotland and north-east England, with the Met Office issuing severe weather warnings.
Commuters were also warned of widespread icy roads in Wales, north-west England, the Midlands and the South.
Weather experts said temperatures would struggle to rise above freezing, meaning large accumulations of snow, which left many people stuck in their homes yesterday, would fail to clear.
Chaos was again expected on the roads and on rail networks today with the freeze showing little sign of relenting.
Aisling Creevey, forecaster at MeteoGroup, the weather division of the Press Association, said: "The majority of the country is already covered in ice and it will be a challenge again today even in areas where roads have been treated due to the low temperatures."
Ms Creevy said today would be a dryer day but temperatures would stay "bitterly" cold into the end of next week, with more snow on its way.
Yesterday the icy conditions claimed at least another three lives - a girl in her early teens was killed in a sledging accident and a mother and her 10-year-old son died in a road crash.
Meanwhile, Top Gear presenter Jeremy Clarkson and socialite Jemima Khan were among those left stranded after plunging temperatures crippled the transport network.
Broadcaster Clarkson was among those forced to abandon his car on an 18-mile journey from Oxford to his own home as snow brought vehicles to a standstill in Oxfordshire, with falls of up to 15cm.
"It was very bad out there," he said. "I wasn't stuck but everyone around me was. I thought, 'I'm not going to get home tonight and it's going to get dark'."
The AA said it dealt with 16,000 calls yesterday, compared with 9,000 on an average Saturday pre-Christmas.
Train services continued to be hit today, with operators in the south, including Southeastern, South West Trains and Southern Railway, running reduced services.
A Department for Transport spokesman said: "The weather over the last 24 hours has been exceptionally severe. Government continues to monitor all aspects of the situation."
Transport Secretary Philip Hammond spoke to representatives from the Met Office, the regions, devolved administrations and other government departments yesterday to ensure everything possible was being done, the spokesman said.
Mr Hammond also held talks with Mayor of London Boris Johnson.
According to the Met Office, snow falls this months have been the heaviest for December in nearly 20 years.
Forecaster Helen Chivers said: "You have to look back to December 1981 to find similar snow depths."
If the second half of the month is as cold as the first, this will be the coldest December on record since 1910.
The sporting world also fell victim to the bad weather as race meetings were abandoned and football fixtures cancelled.
A Gatwick spokeswoman said although the airport remained open today, the weather's impact on airline operations across Europe would inevitably lead to delays and cancellations.
"Today is Gatwick's busiest day of the festive getaway and we are doing everything we can to get passengers on their way and aircraft in the air," she said.
"However, passengers must check with their airline before setting out for the airport to reduce congestion and avoid disappointment. We also advise that passengers check with transport operators before planning their journey to the airport."
Passenger Sue Kerslake and her three young grandchildren spent the night on a terminal floor at Heathrow after their flight to Hong Kong was cancelled.
She told the BBC she arrived at the airport at 8am yesterday, before sitting on a Cathay Pacific plane for seven hours before being told it was not taking off.
"We were told to get off the plane at 7.30pm and no one from the airline has dealt with us since," she said.
"There were thousands of people in departures overnight and it got quite intense at times. The bars were open and some people were drinking and got quite nasty."
Hundreds of stranded people also spent the night at Gatwick Airport, although flights started to recommence this morning.
Meanwhile Southeastern, Southern and South West rail operators said they planned to operate a normal Sunday service, although trains may be subjected to short notice alterations due to the weather.
Elizabeth Herridge was told her KLM flight from Heathrow to Amsterdam was going ahead as expected this morning, but was turned away from Terminal 4 when she arrived.
Ms Herridge, a communications worker, who was hoping to get away for Christmas, said: "I received a text alert from Heathrow at 5.30am this morning saying my 8.40am flight was running on time.
"I didn't ignore it and turned up first thing, but I was greeted by a member of staff at the terminal doors asking why I was here as there were no flights coming in or out and to go home.
"There seems to be a lot of confusion and I have only seen one Heathrow worker. All the airline desks are shut because it is a Sunday - it's absolutely ridiculous."
A tanker which overturned on the M25 near Clackett Lane Service at about 9am today was adding to the woes of motorists affected by icy conditions.
The tanker, which was carrying liquid petroleum gas (LPG) was travelling clockwise when it overturned, Kent Police said.
The M25 is closed in both directions between junctions five and six while the LPG is cleared and the tanker recovered.
A police spokesman said: "It is likely to be closed for some time and motorists are being advised to expect long delays."
Passengers set up camps at Heathrow Terminal 5, with their stacked suitcases and trolleys huddled around improvised beds made of clothes and roll mats.
People dozed off, fully dressed in their warm clothes, or they read, played games or spent time on laptop computers.
Janos Kalman, 50, from Szeged in Hungary, arrived at Terminal 5 yesterday morning at 11am for a flight to Budapest.
The psychiatrist, who had been at Oxford University for two days attending seminars, slept on the cold floor of the terminal building, missing out on the free blankets and mats that airport staff handed out.
He said: "When I realised the flight was cancelled I tried to re-book myself.
"I hired out some time on the internet but the pages were so busy and I didn't manage.
"After that I tried on the telephone, but everything was so busy.
"Finally I queued for one and a half hours and managed to re-book for today, but that was cancelled too.
"Now my colleagues in Hungary are trying to get me on a flight."
The doctor has been forced to cancel appointments with patients, and is now hoping to get home for Christmas and his 25th wedding anniversary next week.
He said: "I'm hanging in there. I'm reading, and trying to enjoy myself.
"I have decided not to go to the hotel because I think it makes things more difficult.
"At the very beginning I was a little worried but now it's OK."
He said he even saw the experience as a kind of experiment, saying: "This is a sort of camp and an emergency situation.
"I've seen people crying and panicking, and the staff trying to cope with it all."
Kiri Meili, 20, from Bangalore in India, arrived at T5 yesterday for a British Airways flight home.
She was given a blanket and mat in the evening to make her night on the floor slightly more bearable.
The student said: "I arrived at 10am yesterday but everything was cancelled. But I just thought I'd hang around in case anything changed."
After one night sleeping at the terminal, she has now decided to try to find a hotel room somewhere near the airport, and is waiting for a friend to pick her up.
"At first I was a bit peeved that it was just BA flights that got cancelled, but then I told myself to get a grip."
Kent Police said later that the full length of the M26 and the M25 between junctions five and six were closed in both directions due to the LPG tanker overturning.
Diversions are under way on the A25 and A21. Motorists heading between Kent and the border with Surrey are being told to expect long delays.
No-one was injured in the incident, a police spokesman said. He added that motorists should only travel if "absolutely necessary" due to icy roads.
Alan Wilcock, RAC patrol ambassador of the year, said: "We're expecting another very busy day today as temperatures remain exceptionally low. The outlook for tomorrow morning's rush hour commute is equally bad - with the cold set to play havoc with cars and roads yet again.
"If you haven't used your car over the weekend and you're planning to tomorrow, it really is worth starting it for five to ten minutes this evening - staying with the vehicle as it's running. It may well be the difference between getting off your drive and calling us in the morning.
"If you do get off your drive, it's important that you stay on the main treated roads wherever possible. It may be tempting to take a shortcut to beat the traffic, but it's also likely to be far more treacherous."
The AA had attended more than 3,000 breakdowns by 10.30am, with calls coming in at 800 every hour.
By the end of the day it expects to have attended around 14,000 call-outs, compared with around 7,500 on a normal Sunday.
The breakdown service said particularly busy areas were along the M40 corridor from Warwick, through Oxfordshire and down into London.
Edmund King, AA president, said many roads were "still treacherous".
"There are thousands of roads with compacted snow which, on hills at minus temperatures, are like ski jumps," he said.
"Our patrols have reported a lack of grit in many areas particularly on some key regional routes. We urge the Highway Authorities to plough and grit as many roads as possible so that we can keep Britain moving.
"We are experiencing more road closures and blockages caused by jack-knifed trucks or trucks stuck in the snow.
"On motorways and dual carriageways we would urge truck drivers to keep to the left lane to avoid blocking the whole route. It might also be worth more HGVs considering snow chains."
A spokesman for Heathrow airport apologised to passengers for today's disruption, and warned: "There will be further cancellations and delays tomorrow as we continue to clear snow and ice, as well as in the days that follow as airlines move diverted aircraft and crew back to their normal positions."
He went on: "We ask passengers to check with their airlines before travelling to the airport, and we will provide regular updates and airline contact numbers through heathrowairport.com.
"This morning we listened carefully to the advice of our airside operations team and reluctantly judged that while Heathrow's northern runway remains clear, the change in temperature overnight led to a significant build-up of ice on parking stands around the planes and this requires parts of the airfield to remain closed until it is safe to move planes around."
National Express was running a reduced services to Gatwick from all areas.
Trips between London and Paris were also suspended because French police would not allow coaches to pass through Calais, a spokesman said.
The earliest possible departures to France will be tomorrow.
The M25 between junctions five and six reopened after the overturned tanker was cleared at around 4pm, seven hours after the accident occurred.
A Kent Police spokesman said there was a backlog of vehicles as traffic slowly cleared from the area.
Anger was growing at Heathrow later as frustrated passengers faced another uncomfortable night on terminal floors.
Trevor Taylor, who had been waiting with his wife and two young sons for a flight for Singapore for two days, described conditions at Terminal 5 as "like a third world country".
The 37-year-old from Basingstoke in Hampshire said: "It's absolute mayhem. There are kids that haven't been fed, there are elderly people in wheelchairs getting cold. It's 'everyone fend for yourself'.
"Frustration is building up. I've been sleeping on a knobbly marble floor and every space you can see is taken," he said.
Although passengers were advised to leave the airport if their flight had been cancelled, treacherous conditions on the roads and fully booked hotels meant some had no option but to stick it out at the airport, he said.
Some passengers said they were running out of money, and although British Airways had been handing out sandwiches, many were craving hot meals.
Others reported lengthy queues for toilets and plug sockets, where stranded passengers had to take turns to recharge their mobile phones.