Flights were supposed to resume Saturday evening, but the airline later announced that none of its planes would leave the airports before Sunday.
Earlier this month, the British Airways website was offline for many hours, while its new "FLY" check-in and flight management system has also suffered major outages. It didn't say what was causing the problem but said there is no evidence of a cyberattack. Airlines depend on huge, overlapping and complex IT systems to do just about everything, from operating flights to handling ticketing, boarding, websites and mobile-phone apps.
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 canceled flights.
Affected customers can claim a full refund or rebook to a future date for travel up until the end of November 2017, it said.More news: Remembering the Fallen at Arlington
"We would like to say again how extremely sorry we are for the inconvenience this is causing customers during this busy holiday period".
The airline said its engineers were "continuing to work hard" to restore its services and it hoped most United Kingdom flights would resume on Sunday.
British Airways on Saturday cancelled all flights from London's Heathrow and Gatwick airports after a global information technology failure.
"We knew there was problems in London, but the internet was showing that it was still OK to book it", traveler Sandra Hewitt said.More news: Spieth right back in the mix at Memorial
"We are continuing to work hard to restore all of our IT systems and are aiming to operate a near normal schedule at Gatwick and the majority of services from Heathrow on Sunday", BA said in a statement.
Travelers on this spring bank holiday weekend in Britain were already reporting disruptions, including baggage and flight delays, when the airline reported serious computer problems around midday.
Some travellers said they were not informed that their flights had been cancelled until more than an hour after the airline told the media. We advise passengers to check the status of their flight prior to travelling to the airport. Some BA flights were still arriving at Heathrow Saturday afternoon, while many were listed as "delayed".More news: Real Madrid's campaign in the Champions League
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.
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