British Airways resumed some flights from Britain's two biggest airports on Sunday after a global computer system failure created chaos, but hundreds of passengers were still waiting for hours at London Heathrow.
BA said it aimed to operate the majority of services from Heathrow and a near normal schedule from Gatwick, the capital's second busiest airport. Heathrow, however, said it expected further delays and cancellations of BA flights.
At Heathrow's Terminal 5, where BA is the dominant carrier, hundreds of passengers were waiting in line on Sunday and flight arrival boards showed cancelled flights.
Some passengers were curled up under blankets on the floor or sleeping slumped on luggage trolleys. Several passengers complained about a lack of information from BA representatives at the airport. Others said their luggage had been lost.
"Many of our IT systems are back up today," BA Chairman and Chief Executive Alex Cruz said in a video posted on Twitter.
"All my British Airways colleagues on the ground and in the air are pulling out all the stops to get our operation back up to normal as quickly as we possibly can, we're not there yet."
Cruz said BA, part of Europe's largest airline group IAG, planned to fly all its long haul services from Heathrow on Sunday, although there would be delays due to the knock on impact from Saturday's disruption and some short haul flights would be cancelled.
He also asked passengers not to arrive at Heathrow too early, warning they would not be admitted into Terminal 5 until 90 minutes before their flight's scheduled departure time.
Gatwick and Heathrow also told passengers not to travel to the airports unless they were rebooked on other flights.
British Airways cancelled all its flights from Heathrow, Europe's busiest airport, and Gatwick on Saturday after a power supply problem disrupted its flight operations worldwide and also hit its call centres and website. Cruz said there was no evidence of any cyber attack.
A spokeswoman for BA could not immediately detail the exact number of flights cancelled on Saturday.
HAOS FOR PASSENGERS
Thousands of passengers queued for hours in departure halls at the airports on a particularly busy weekend. Monday is a public holiday and many children were starting a one-week school holiday.
Cruz said those who decided not to fly could rebook for dates until the end of November, or receive a full refund.
While British Airways could face a one-off financial hit from the cancellations, the risk to its reputation among customers could have a more damaging longer-term effect.
It is already facing declining customer ratings following unpopular decisions made as it faces competition from low-cost airlines. These include starting to charge for food on short haul flights last year to cut costs.
"There will be short term financial repercussions of this outage in terms of lost revenue, compensation for passengers and cost of alternative arrangements," said Kunal Kothari, UK All Cap equity analyst at Old Mutual Global Investors, one of IAG's top-10 shareholders.
"I do not, however, expect the outage to have lasting financial repercussions for the group."
Terminals at Heathrow and Gatwick became jammed with angry passengers, with confused BA staff unable to help as they had no access to their computers, according to passengers interviewed by Reuters.
Some passengers expressed frustration on Twitter over missing bags and long waits in telephone queues to speak to BA staff. BA said it had introduced more flexible rebooking policies for passengers affected.
While other airlines have been hit by computer problems, the scale and length of BA's troubles were unusual.
Delta Air Lines Inc cancelled hundreds of flights and delayed many others last August after an outage hit its computer systems. Last month, Germany's Lufthansa and Air France suffered a global system outage which briefly prevented them from boarding passengers.