British Airways is to cancel all UK domestic services from London Heathrow on Thursday following severe fog.
British Airways said the disruption could last for 48 hours
The firm said a significant number of European flights, including routes to Paris and Brussels served by the Eurostar, will also be grounded.
BA and other airlines cancelled more than 180 flights to and from Heathrow on Wednesday.
Some 18 Gatwick flights were affected as were services from Liverpool John Lennon and Cardiff International.
Airport operator BAA said 223 flights had been cancelled altogether.
BA said it plans to run about 220 shorthaul flights to and from London Heathrow on Thursday, out of a normal daily total of 400 shorthaul and domestic flights.
BBC transport correspondent Tom Symonds said BA had opted to cancel flights served by train services.
He added that thousands more passengers than Wednesday - about 20,000 more in total - are expected to arrive at Heathrow on Thursday.
The BBC Weather Centre says the poor conditions are set to continue for the next few days when many people will be travelling for the festive break.
It warned that the weather was likely to cause problems for motorists in England and Wales right up to Christmas.
The fog has also caused delays for ferries docking at Pembroke port - with 160 passengers on Irish Ferries' Isle of Inishmore coming in 10 hours late.
The BA flights affected on Wednesday were in both directions between Heathrow and Manchester, Edinburgh, Glasgow, Aberdeen and Newcastle.
Long-haul flights were still being operated by the airline, but with severe delays.
British Airways customers should contact 0800 727 800 to see if their flight is still operating
The airline said dense fog and low visibility had caused air traffic control to place restrictions on flights landing and taking off.
Flow rates were at 50% of normal capacity throughout Wednesday morning, affecting all carriers at the airport, after the spacing between planes in the air was increased from three miles to six miles.
A Heathrow spokesman said Lufthansa and Alitalia were among the other airlines affected.
At Cardiff International Airport, a number of flights were delayed or diverted to Bristol.
Several flights, including Easyjet flights, in and out of Liverpool John Lennon Airport were also cancelled.
BA is urging customers due to travel on domestic flights to or from Heathrow not to travel to the airport on Thursday.
It said those on any cancelled service are able to rebook, re-route their ticket subject to availability or claim a refund.
Friday is expected to be the busiest day for air travel
BBC weatherman Darren Bett said: "The reason we've got fog is a large area of high pressure over the UK and trapped underneath is moist, low-level air which produces this fog and low cloud.
"The same area of high pressure will still be sitting over England and Wales on Christmas Day, so right until Christmas and even beyond there's the risk of fog."
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.
The peak day for travel is expected to be Friday.
BAA estimates two million people will be leaving through its seven UK airports, while about 230,000 are set to travel across Channel, and more than 125,000 are booked on Eurostar train services out of London's Waterloo.
Are you stuck at the airport? Will you miss your Christmas flights because of the fog? What assistance are you getting from the airlines or airport staff to make the delay more comfortable?
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I've been stuck in Cardiff International for the past three hours - I was meant to be in Glasgow by now! So much for festive spirit!
Stephen Lee, Cardiff International
It's surprisingly quiet in the departures lounge in Terminal Four, considering the long delays. Information is hard to get, but with the fog lifting and returning all the time it must be a nightmare for the customer services staff. BA just keep handing out snack vouchers to keep us happy.
Christine, LHR T4
I have to say I was worried having seen the problems in August but the people at the airport have kept me up to date. It's not there fault and seems so unfair to keep knocking BA for things that are out of their control.
It's not just BA. My sister was due to fly from Aberdeen to Heathrow with BMI tonight ahead of our flight out to Oman to visit our parents for Christmas. Seeing all the BA cancellations, I rang BMI to see if her flight was affected and was assured that it wouldn't be cancelled. I queried this as all BA flights had been grounded, and pointed out that hers was the last flight to London today - if they cancelled it later tonight, she'd be stuck in the UK for Christmas. Again, I was reassured that things would be fine. Two hours later, and BMI have just cancelled her flight. She's now on her way to the railway station, having had to leave work early. I realise the weather is out of their control, but they shouldn't be offering their customers the kind of assurances I was given when they clearly are lacking any up-to-date information themselves.
Simon Allum, London, UK
I'm supposed to be flying out from Norwich today, but half their flights have been cancelled and the other half are delayed. This fog's supposed to continue for the next three or four days and I'm now looking at the trains as a contingency plan! Unfortunately, that will cost me at least twice as much, and take four times as long! It all reminds me of last year when it took me three attempts to get back to Norwich after Christmas due to snow and fog. Anyone managed to invent a teleporter yet?
Jenny, Norwich, Norfolk
My parents were supposed to be flying out here from Gatwick this morning to spend Christmas with us, but their flight was cancelled. They are now booked on a flight this afternoon, but will change in Brussels. Hopefully they will make it out here tonight, especially has they are brining out lots of presents for our children.
Ashley Holland, Alsace, France
I'm stuck in Bristol. Was due to fly out to Newcastle at 9pm, but now I'm going to miss it. I wonder if I'll get any compensation. I don't suppose so, I think 8000 of us are affected here.
Air France flights from Paris-Charles de Gaulle Airport are also affected. Flights this afternoon have been cancelled at the last minute, with passengers being left in the dark by airline employees. My 1pm flight to London was cancelled just two minutes before it was set to board, and there is a long backlog of passengers trying to find a way home. The airline is now telling passengers that they will most likely not be travelling to London today and making tentative arrangements for tomorrow. What a lovely way to spend the holidays!
David Hanson, London, UK