Dense fog is continuing to cause misery for Christmas holiday travellers at Heathrow and other UK airports.
Some passengers stayed overnight
Hundreds of domestic flights and some international flights are grounded.
Heathrow has been the worst hit, with 40,000 people affected, and services from Gatwick, Manchester, Glasgow and Cardiff have also been disrupted.
Passengers voiced anger at a lack of information. Fog is forecast on Friday, when BA says all domestic flights to and from Heathrow will be cancelled.
About 350 flights out of Heathrow were cancelled on Thursday, a 40% reduction in services.
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Napil Berry, 37, from Nottingham, said he had missed his sister's wedding in Stockholm.
Sitting outside the terminal concourse, he said: "I told my sister I'm not going to make it. She said: 'Try to come in time for Christmas.'
"I'm not angry with BA about it but I am angry and upset I missed my sister's wedding."
The BBC's Lucy Rodgers reported that hundreds of passengers in Terminal One had frantically been making alternative arrangements, while others were reading, talking or sleeping in a marquee erected outside as they waited for news of flights.
BBC News website business editor Tim Weber, who was flying to Germany with his family for his father's 80th birthday, said hundreds of passengers were queuing outside Terminal Two.
He described the situation as "absolute mayhem" because travellers were only being allowed into the terminal an hour before their flights were due to depart.
"Women with children, including babies, are standing outside in the cold because the marquee is chock-a-block with people," he added.
The disruption will continue on Friday, with British Airways announcing it would be operating no domestic flights in and out of Heathrow and a reduced short-haul schedule because of the fog.
About 100 flights in total have already been cancelled.
BBC weather forecaster Darren Bett said "right until Christmas and even beyond there's the risk of fog".
Low visibility had caused air traffic control to place restrictions on flights landing and taking off at Heathrow.
The weather also affected services from Edinburgh and Norwich. Some services from Coventry Airport operated from East Midlands Airport and Birmingham International.
BA said 200 flights, including European short-haul and domestic, had been cancelled in total.
The firm said about 3,000 passengers were put on coaches from Heathrow to Manchester, Newcastle, Glasgow. Edinburgh and Aberdeen.
BMI said about a third of its services in and out of Heathrow - some 40 flights - had been cancelled because of the reduction in air traffic movement capacity.
Some flights at Northern Ireland's main airports that were unaffected by cancellations saw delays of up to three hours.
BA said the fog would also hit Heathrow flights on Friday
Many passengers on cancelled flights are choosing to travel by rail instead and a spokesman for the train operators said they were coping with the extra passengers.
The Association of British Travel Agents said about three million people are set to go abroad over Christmas and New Year, with southern Spain, north Africa, Paris, Amsterdam, Dublin and New York among the top destinations.
Meanwhile, the RAC said "an estimated 18m people" were expected to be driving on Britain's roads over the next few days.
Road information company Trafficmaster said congestion began to build up soon after lunchtime and widespread traffic jams were reported on many major routes.