The computer system used by British Airways suffered a "major" global failure on Saturday (27 May), causing all the day's flights from Heathrow and Gatwick to be cancelled.
The outage came as thousands of passengers were set to travel over the bank holiday. Many criticised the carrier's lack of communication as "stunning".
The airline said extra staff would be at the airports and advised passengers booked on flights to stay home.
"Please do not come to the airports. We have experienced a major IT system failure that is causing very severe disruption to our flight operations worldwide," a statement by the airline said.
"We are extremely sorry for the inconvenience this is causing our customers and we are working to resolve the situation as quickly as possible."
The airline's website also appeared to malfunction but was later fixed.
There were long queues at the airports in London and customers complained that they were not told what was happening.
Brent Hoberman, co-founder of lastminute.com, waited more than 5 hours for a flight to Nice, which was among those cancelled. He told IBTimes UK the "lack of communication was simply stunning".
Hoberman said British Airways staff, including plane captains, had "no idea what was going on" and gave passengers conflicting information. He claimed the BBC announced the flight cancellations before airport staff were told and departure boards were not updated.
"I think they need a backup plan here," Hoberman said, adding that the queues just to leave the airport were taking hours.
In Nice, southern France, Michael Taylor, a British chartered engineer, said there had been "no information" and the situation was "jolly poor".
He told IBTimes UK that despite having the British Airways app, he received no advance warning of the situation that could have enabled him to re-book his flight. On arriving at the airport, he was told not to worry because a backup system was in place but BA flights were described as "delayed" as their departure time approached.
"One staff member came over but they didn't seem to know what was going on," Taylor said.
Passengers also reported being made to wait on planes while the crew attempted to deal with the situation.
One pilot described the situation as "catastrophic", according to passenger Martyn Kent, who said he was stranded on his plane for one and a half hours though he was able to continue his journey later.
On social media, numerous passengers at various airports complained about IT failures at British Airways desks.
British Airways has said there is no evidence the outage was caused by a cyber attack. Earlier this month, ransomware called WannaCry paralysed computers in more than 200,000 organisations across 150 countries, including dozens of NHS trusts in the UK.
A Heathrow Airport spokesman was aware of an issue with British Airways planes but did not elaborate.