Holidaymakers travelling with British Airways are facing delays and problems checking in online after an IT glitch.
At least 81 flights have been cancelled at Heathrow Airport, with 10 cancelled at Gatwick Airport. More than 200 other flights have been delayed.
BA said it is 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 re-book 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 outage, but a problem with 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.
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”.
Meanwhile speaking to BBC News, Kieran Healey-Ryder, from Glasgow, said he chose to disembark his 0700 BST flight from Glasgow to London following a two-hour delay.
“We boarded at about 06.30am 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.
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, phone us or messaged us about the delay,” he said. “We’re just supposed to know what to do. I’m really angry with them.”
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.
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