The UK's flag carrier has been "working hard to restore" its global systems after a computer meltdown grounded flights. British Airways was also trying to reunite bags lost at Heathrow with their disgruntled owners.
Travellers on British Airways were still suffering considerable delays on Sunday - the day after a system-wide computer meltdown grounded all of BA's travel at its London Airports and left tens of thousands stranded. London's Heathrow Airport said early on Monday that there were still some disruptions to BA flights from the airport.
On Sunday night the company's website admitted a "major IT system failure."
"We are working hard to restore our global IT systems," officials said. "The majority of flights are now operating but we are experiencing further disruption."
"Our terminals at Heathrow are still congested so we ask that you do not to come to the airport unless you have a confirmed booking for today and know that your flight is operating," the company added. At Gatwick airport south of London, a "near-full operation" was being run on Sunday.
Passengers were asked not to turn up at the terminal until "90 minutes before your flight's scheduled departure time." About 200 BA flights in and out of Heathrow are believed to have been cancelled on Sunday.
For passengers due to fly on Monday or Tuesday, the company said that those who did not want to travel could get a "full refund or rebook up to the end of November."
Passengers interviewed at the crowded airports waiting for a flight complained about the paucity of information they were being given by British Airways.
Lots of baggage
There are also a "significant number of bags at Heathrow which we will be reuniting with customers via couriers as soon as we can" British Airways announced. There will be no charge for delivering the bags.
On Saturday, the company was forced to cancel all of its scheduled flights from London's Heathrow and Gatwick airports. Some British media have speculated the system failure was the result of a cyberattack but British Airways rejected that claim.
No cyberattack, no back-up?
The airline said it had found "no evidence that it's a cyberattack" and the problem is believed to have been caused by a power supply issue. A number of commentators have expressed surprise there was apparently no back-up system.
The GMB trade union has blamed job cuts and the outsourcing of IT work to India after BA hired Tata Consultancy Services to run systems. The union wrote to then Home Secretary Theresa May four times last year, claiming that outsourcing systems was of concern for the country's future safety.
Industry experts are suggesting that British Airways may have to pay more than a hundred million euros in compensation to passengers for the delays and cancellations.
"We would like to say again how extremely sorry we are for the inconvenience this is causing during this busy holiday period," BA said on its website on Sunday.
ls/jm (Reuters, AFP, AP, dpa)