Holidaymakers travelling with British Airways are enduring cancellations and delays after an IT glitch.
At least 117 flights have been cancelled at Heathrow Airport, with 10 cancelled at Gatwick Airport. More than 200 other flights have been delayed.
BA said it was reverting to manual systems for check-in at airports.
It has apologised for the disruption and said customers on short-haul services from Heathrow, Gatwick and London City can rebook another day.
"A number of flights continue to operate but we are advising customers to check ba.com for the latest flight information before coming to the airport," it said, adding that passengers should allow extra time.
The airline said the issue was not a global problem, and involved two separate systems - one which deals with online check in, the other that deals with flight departures.
The issues are affecting flights across its network but not at every airport, it added.
Passengers have been invited to rebook flights on any other day up to next Tuesday.
Travellers have been expressing their frustration on social media.
One holidaymaker at Gatwick Airport tweeted that he was stuck on the ground because the "pilot can't get data".
Another disgruntled customer at Manchester Airport said he was meant to be on the BA1385 flight, but was told at the gate that there was a "worldwide British Airways system outage".
Speaking to BBC News, Kieran Healey-Ryder, from Glasgow, said he chose to disembark his 07:00 BST flight from Glasgow to London following a two-hour delay.
"We boarded at about 6.30am and pretty quickly they said there had been an IT outage across their network and they were unable to depart," he said.
"Unfortunately it doesn't look like we'll be making it down to London today," he added.
Alex Brayson and his partner faced delays from Newcastle Airport en route to their wedding near Lake Bled in Slovenia on Saturday.
Despite boarding their first flight shortly before 06:00 BST, Mr Brayson said they missed their connecting flight to Venice, and their onward transfer was running three hours behind.
"We've got 25 of us flying to the wedding so we are growing anxious," he told BBC Newcastle.
"Me and my partner booked the trip so we've had to organise everything again."
Sam Angeli is stranded in Cyprus with his wife and two children after his flight home to London on Wednesday evening was cancelled.
He has been forced to spend more than £850 on two rooms at a hotel for the night.
Mr Angeli said his experience of BA had been an "absolute shambles".
"They've not emailed us, phoned us or messaged us about the delay," he said.
"We're just supposed to know what to do. I'm really angry with them."
At the scene
There are lots of families travelling with children who are very annoyed, with many reporting that they can't get in touch with BA.
We've had people handing out bottles of water, anything that can help with the long wait.
Passengers are being handed out leaflets about compensation.
They have been told to leave the airport and either go home or find a nearby hotel for which they will be reimbursed.
That's all to try and get those queues down.
Holidaymakers and rush-hour commuters have also faced delays and cancellations due to three fires which broke out next to railway tracks near Victoria Station.
Southern said the fires had been put out and lines reopened but residual delays are expected to last until about 14:30 BST.
This is not the first time BA has experienced IT problems, as it suffered major computer failures over the spring bank holiday weekend in May 2017.
On that occasion, the airline cancelled 726 flights and tens of thousands of passengers were left stranded.
It has been a problematic year for British Airways which is being threatened with strikes by pilots.
It is also set to be fined millions of pounds over a cyber attack on its security systemsin which the personal data of up to 500,000 customers was stolen.
What rights do affected passengers have?
BA has a duty of care to passengers whose flights have been affected. Rights include:
If the flight is cancelled, the airline must get them on an alternative flight to their destination.
That might be a re-routed flight, perhaps including a stopover, or the next available direct flight. They can put customers on a flight with a different airlineSome passengers will choose to take a refund instead, but the airline's duty of care ends at that moment. That means someone who takes the refund and books their own - more expensive - alternative flight is unlikely to have the difference reimbursedIf a flight is still expected to depart, but late, then passengers will initially have to wait, but after a certain amount of time the airline should offer food, drink, and - if necessary - overnight accommodation. For delays of more than five hours, passengers get the same rights as they would if the flight was cancelled
In this case - when the disruption appears to be the fault of the airline - passengers may well be entitled to additional compensation under EU rules. This requires passengers to make a claim to the airline, and the level of payout depends on the type of flight and the length of delay.
More information for passengers is available on the BA website, and passenger rights are explained in detail on the Civil Aviation Authority website.