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.”