Forecasters say the UK is experiencing some of the most severe winter weather in a century, with more sub-zero temperatures and snowfalls of up to 20cm, or eight inches, expected today.
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Temperatures plunged to a record low for Northern Ireland in Castlederg, County Tyrone, which saw lows of minus 17.6 degrees Celsius last night. The lowest UK temperature overnight was thought to be minus 19.6 in Chesham, Buckinghamshire.
Driving organisations have warned of "potentially fatal" conditions on the roads, as many people hope to make an early start to their Christmas holidays.
Only around 20 flights were able to take off or land at Heathrow on Sunday, out of 1,300 flights the airport sees on a usual day. A "limited schedule of arrivals and departure" has been in operation from the airport since 6am this morning, but passengers are warned that disruption is expected for days to come.
Gatwick Airport was open, with operations "returning to normal" but passengers have been warned to contact their airline before travelling as delays and cancellations are expected to continue.
Mayor of London, Boris Johnson, said he questioned BAA chief executive Colin Matthews about the situation: "I stressed the huge economic importance of Heathrow. If there was a war on, we’d surely be able to sort this out," he said.
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"I also expressed my hope that they would pull out all the stops to ensure that the planes get moving again.
"Most people realise that it has not snowed at Heathrow for some time, so it is vital everything is done to get the aircraft and passengers moving again."
A statement on the airport’s website said there would continue to be disruption "as airlines move diverted aircraft and crew back to their normal positions and we continue to manage the impact of the poor weather."
If there was a war on, we’d surely be able to sort this out. London Mayor, Boris Johnson
British Airways said Heathrow was only able to use one of its two runways and "many areas of the airfield remain unusable, including areas around parked aircraft".
A British Airways spokesman confirmed that the situation is difficult: "Most of our aircraft are still frozen. With incredibly low temperatures at Heathrow, it is difficult to get anything moving."
Asked about complaints about a lack of information over the weekend for air passengers, BAA spokesman Andrew Teacher admitted there had been difficulties.
"There have been problems and we are extremely sorry for those, and that is something we will obviously have to look at, along with airlines, about how we communicate. The issue we have at Heathrow is that while there are 77,000 people working there, they are not all employed by BAA, so there is a limited degree to which we can control things.
"Obviously we are not trying to shun responsibility, we are trying to look at what we are doing and focus on the situation now.
"We are obviously not going to sit here and claim that it has gone well because it has been a very troubling time for many, and we are extremely sorry for the disruption to people’s journeys."
One passenger told Channel 4 News there are "thousands and thousands of people" at Heathrow’s Terminal 3 alone.
"They are standing shoulder to shoulder," Paul Lomax said. See below the picture he put on Twitter a short time ago.
He and his wife were trying to travel to Canada via New York on Saturday, but their flights have been delayed.
Mr Lomax said the major problem was the lack of information. He and his wife had been queuing for three hours today when they were told that they should go to a different part of the terminal. But he said that it is hard to get official information.
"There is no real organisation," he said. "They are trying the best they can, but there are not enough staff.
"We are being told to check with the airlines for more information, but nobody can get through on the telephone. The only way you can speak to someone is to come to the airport.
"There are so many people, they are everywhere, as far as the eye can see. I heard a rumour that earlier this morning it was so busy they were not letting people through the door.
"There are queues everywhere, and you have to ask people what they are queuing for before you join them. There are no signs, no barriers, and nobody organising the queues.
"A person at the front will speak to someone, then pass the information down the line."
He said that most people are dealing with the situation well: "The majority of people are taking it in their stride. A few people are getting frustrated, but that is because they have been sleeping on the floor with no blankets and very little food."
The snow is also a problem for passengers trying to get back to the UK.
Sarah Corp tried to travel from Prague to Gatwick on Saturday.
"My flight was cancelled via text message at about lunchtime," she told Channel 4 News. "Yesterday it was more difficult to assess whether my evening flight would run as Gatwick, Prague and Easyjet said it was still scheduled (if delayed) until the time I left for the airport. With Gatwick open and flights running, I assumed I would be able to get home. I was in a taxi on the way to the airport last night when I received a text from Easyjet saying my flight was cancelled."
She added "it is interesting that in Prague, where we have had a lot of snow and temperatures have consistently been below zero for weeks, that the airport has not closed at all, and passengers are suffering from minimal delays."
Transport Secretary Philip Hammond has promised there will be an inquiry into the way stranded passengers were treated at Heathrow over the weekend.
"Once we have got through this problem, once we have got things moving again, then we will have to have a discussion and find out exactly what went wrong.
"I think whilst people are obviously deeply upset about the inconvenience, particularly at this time of year, of having their travel plans disrupted, most of what I am hearing is a sense of outrage about the way they were then treated when they were stranded at Heathrow airport."
Eurostar services have also been affected by the weather, with an emergency timetable in operation for the rest of the week. A spokesman urged passengers not to travel unless it was absolutely necessary, as speed restrictions are adding up to two hours to journey times. The company is offering refunds or rescheduled tickets to people who could postpone their journey.
Train operator East Coast has warned passengers there will be longer journey times and cancellations throughout the day. Yesterday all services were suspended between King's Cross and Peterborough due to damage to overhead power lines.
Network Rail has said most routes are expected to run normally during the day, but some companies in the South East are going to operate amended timetables.
We are obviously not going to sit here and claim that it has gone well because it has been a very troubling time for many, and we are extremely sorry for the disruption to people’s journeys. BAA Spokesman
More to come
Severe weather warnings have been issued for south west England and Wales today, with the Met Office saying between two and four inches, or five to 10 cm, of snow will fall in many places, with up to eight inches, or 20 cm, on higher ground. London is expected to see snow in the afternoon, while freezing fog is expected in areas of northern England.
There are warnings from forecasters that low temperatures are expected to continue over the coming days, with more snow expected on Wednesday.
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