Christmas travellers are facing severe disruption as heavy snow falls across many parts of the UK.
Airport closures have hit thousands of passengers. Heathrow expects some flights to depart from 0600 GMT, while many Gatwick flights remain cancelled.
Roads, railways and airports are affected across the UK, amid severe weather warnings for many areas.
Airports in Germany, France, Italy and the Netherlands are also suffering cancellations and delays.
The snow is forecast to continue in many parts of the UK, with up to 25cm expected in some areas of England, and a number of severe weather warnings are in place.
Heathrow Airport had closed on Saturday but some flights are expected to depart later, although there will be no arrivals until 1000 GMT at least.
However, the effects of recent snowfall is expected to cause delays and cancellations both at Heathrow and Gatwick.
On Saturday evening Heathrow had announced that it would remain shut until Sunday and urged passengers not to travel to the airport unless they were sure their flight was "definitely operating".
Passengers have been stuck for hours on both incoming and outgoing flights at Heathrow.
Airport operator BAA said it was bringing in water, food and blankets for passengers stranded at airports overnight.
Gatwick reopened at 1445 GMT after 10cm of snow was cleared from the runway but delays and cancellations are expected, and there are no inbound flights for the rest of the day.
British Airways, Easyjet and Aer Lingus have cancelled all flights from the airport for the rest of Saturday, with the exception of the BA2055 to Sharm el Sheikh.
London Stansted's runway has now re-opened, as have Luton and Birmingham airports, but long delays and cancellations are still expected.
There was also disruption at Exeter, London City, Aberdeen, Birmingham, Bristol, Southampton and Cardiff airports.
Scottish airports are open but are being affected by closures elsewhere. Glasgow is starting to receive London-bound flights.
In Wales two health boards are appealing for medical staff to help out over the weekend. Members of the public with 4x4 vehicles have been asked to drive health workers in after many were stuck at home.
Several roads across the country remain closed and bus services were disrupted in the south.
In Northern Ireland freezing conditions continue to make travel difficult following some of the heaviest snowfall for 25 years - with more forecast.
Both Belfast International Airport and Belfast City airports have reopened, although there are knock-on effects from other closures.
Jon Caudwell, from the Highways Agency, said they were doing their best to keep major roads in England clear but needed help from motorists who should "really seriously consider" whether they needed to go out.
He said he was surprised at the level of traffic on the roads, given the advice not to travel. He said in some areas abandoned vehicles and jack-knifed lorries had blocked access for gritters.
BAA tweeted about Heathrow: "There are hundreds of staff who will work right through the night at Heathrow to make the airfield safe for flights tomorrow."
Helen Abel, a solicitor from East Finchley in north London, was stuck at Heathrow on a plane bound for Sri Lanka for almost 11 hours on Saturday. She and her friend are trying to get away for a two-week break over Christmas but on Saturday evening the airline confirmed the flight would not be leaving before Sunday.
Passengers were given a meal on board before being told they could leave the Sri Lankan Airlines plane and spend the night in the airport, or risk going home and not making it back for the flight departing on Sunday.
Speaking from the plane she told BBC News: "It's so frustrating, although obviously it's not the airline's fault. The toilets have become pretty interesting though… there are better places to spend 10 hours."
Another passenger, charity worker Stuart Coles, spent five hours sitting on a Virgin plane from Miami that had landed at Heathrow but could not get to its stand as it had not been cleared of ice and snow.
"There were a lot of Americans and Canadians on the plane who couldn't understand why so little snow had brought everything to a halt," he said.
On what is one of the busiest days of the year for travel and shopping, other problems caused by the weather include:
- The M40, between junctions 4 and 5, suffered delays of around four hours, with BBC Radio 5 live's Nigel Adderley earlier describing it as as "car park with bumper-to-bumper traffic"
- The M5 was the scene of a huge traffic jam on the northbound carriageway, near the junction with the M6, with nothing moving for 20 miles, said the BBC's Colette Hume.
- The AA had gone to more than 11,000 breakdowns by 1730 GMT, with calls peaking at 1,200 every hour - about twice the normal rate.
- Many rail operators are running reduced services, including Southeastern, South West Trains, Southern Railway and First Capital Connect. For rail information the Association of Train Operating Companies has provided a new passenger phone number - 08453 017 641
- Many London Underground lines are delayed after signal point problems caused by the freezing conditions. Overground services are also affected, and Eurostar services are delayed by up to an hour.
- The North West Ambulance Service warned of delays reaching emergency patients
- Brent Cross shopping centre in London was forced to close, angering Christmas shoppers, many of whom had to leave their cars behind
- Lincolnshire County Council said it had already used about 60% of its grit stocks despite starting the winter with 31,600 tonnes - 8,000 more than usual
- A number of weekend sporting fixtures were cancelled affecting Premiership football matches on Saturday and Sunday, Championship and Scottish games, plus rugby union and horse racing
- An urgent appeal is being made for blood donors, particularly those who are O negative, as stocks are running low
- Companies have warned of a backlog of deliveries which may not reach customers before Christmas
- In the rest of Europe some 2,500 passengers were stranded overnight at Frankfurt airport in Germany after flights were cancelled; Florence airport, in northern Italy, is closed and hundreds of cars stuck are on highways in Tuscany; in France, snow has affected road and train travel in the west and north and Paris' Charles de Gaulle airport is reporting delays; in the Netherlands, many flights have been cancelled at Amsterdam's Schiphol airport.
The Met Office warned that heavy snow showers were expected across much of Scotland, Northern Ireland, and parts of north-east England, with up to 20cm of fresh snow possible in places by the end of Sunday.
Elsewhere there were warnings of widespread icy roads following Saturday's snowfall.
Transport Secretary Philip Hammond said major roads were prepared for the "extreme cold weather", which had been predicted, and public transport services were "by and large still working".
But he added that when there is a rapid fall of snow "there is no way you can make it simply go away".
Last night temperatures in England, Wales and Northern Ireland averaged between minus 5C and minus 10C, whilst in Scotland they typically fell to between minus 15C and minus 20C.
Hundreds of drivers were stranded overnight on the M6 motorway in Greater Manchester. Similar conditions in Germany and Italy also saw drivers stuck on icy roads, as temperatures plummeted.