British Airways has cancelled all flights in and out of Heathrow until Friday evening because of unofficial strike action by ground staff.
Passengers face long delays following strike action by BA staff
Some 70,000 passengers have been prevented from travelling after hundreds of BA staff took unofficial action in support of 600 sacked catering workers.
The BBC News website looks at the implications of the strike, the main parties involved, and plans to resolve the dispute.
Q: Who has gone on strike and why?
The dispute began on Wednesday when the company that caters for BA flights, Gate Gourmet, sacked 600 of its staff following an unofficial strike over the company's restructuring plans.
This led to a shortage of catering supplies on BA flights, and a decision was made to ground all Thursday's flights and divert 14 that were inward-bound.
Hundreds of BA baggage handlers, ground staff and loaders - represented by the Transport and General Workers' Union - then went on unofficial strike in support of the sacked catering staff.
Theirs was a secondary walk-out but BA said there was little it could do as the strike was related to a dispute outside its control.
BBC correspondent Russell Hayes said the TGWU recognised the walk-out by BA staff was illegal "wildcat" action and was trying to persuade members to go back to work.
Are negotiations continuing?
Yes. After initial talks, Gate Gourmet said the door was open for further negotiations, but the TGWU has said it could not see a way forward.
Gate Gourmet has said it will not re-instate its staff and told the TGWU it was facing a financial crisis.
But the TGWU has argued that managers provoked the dispute.
BBC correspondent Stephen Cape said the dispute had been "bubbling for some time".
AFFECTED AIRLINE NUMBERS
0800 727 800
0870 000 0123
Sri Lankan Airlines:
0208 538 2000
0845 758 1111
0870 850 9850
British Med Airlines:
0870 850 9850
"Gate Gourmet felt it was losing an awful lot of money each year and they needed to restructure the business," he added.
"They believe that, certainly, they've managed to get rid of some militants in the process and they don't want to take them back again.
"The union say that as far as they are concerned, what happened was completely immoral, it was American-style union-busting tactics, and as far as they are concerned, it was sacking by megaphone."
Despite the apparent stalemate, there have been strong suggestions that further talks will be held later.
How long is the dispute expected to last?
Over at BA, sources said there was "much anger" at the airline and the hundreds of staff on strike may not come back for some time yet.
This means the action could stretch on for the next few days.
BBC correspondent Tom Symonds said: "Sources I've been speaking to at BA say that even if everybody comes back to work today, this could be a three or four-day affair in terms of the disruption that it causes."
However, a mass meeting has been planned involving the 600 sacked staff.
The BBC's Stephen Cape said: "Union leaders will be expecting a fairly rough ride as they try to work out what they are trying to do, but Gate Gourmet has made it clear at this time that they are not intending to reinstate those 600 people.
He added: "Only if Gate Gourmet is prepared to change its position, compromise in some way, can this dispute really move on because if you sort out the Gate Gourmet dispute then naturally the BA staff who are taking action will return to work."
What is BA saying?
BA chief executive Rod Eddington apologised to passengers and called for more talks to begin urgently between the TGWU and Gate Gourmet.
He said BA was forced to cancel so many flights because it did not have "sufficient airport staff to operate flights into or out of Heathrow".
Nearly 100 BA aircraft and 1,000 pilots and cabin crew were in the wrong places around the world, he said.
"We simply cannot mount a robust operation any earlier.
"It is a matter of huge disappointment that we have become embroiled in an issue which is not a dispute with British Airways and disrupted thousands of people who were about to take their hard-earned summer holidays."
He urged the TGWU to encourage its members "to put our customers first and go back to their work places".
What is the current situation at Heathrow?
Many passengers are crammed on to floors inside the Terminal Four departure hall while others were forced to queue outside on Thursday.
On Friday, 550 BA flights were due to leave but it was unclear how many of them would actually depart.
The BBC's Tom Symonds said there was no indication what would happen after 1800 BST on Friday.
"It's a very serious situation for the airline. BA are saying if you are due to fly during the day, the best thing to do is to stay away from Heathrow, call the airline, and hopefully they can advise you."
Are any other airlines affected?
Yes. Sri Lankan Airlines, Finnair, GB Airways, British Mediterranean Airlines and Qantas are affected. They are serviced by BA ground staff.