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.
By Leigh Milner, BBC News, in Heathrow Terminal 5