British Airways suffered a major online service disruption on Friday with passengers complaining of cancellations and delays while digital systems were down.
The airline's website and app were inaccessible for hours on Friday evening, leaving customers unable to book flights.
BA said it was a “technical issue” and that it is still operating flights and checking passengers in. There was no indication it was a cyber attack, BA added.
Britain's National Cyber Security Centre had warned companies and organisations to bolster their online defences against cyber attacks as the Ukraine crisis exploded.
Twitter users posted messages asking BA when the problem would be fixed, with one customer showing a video of lengthy queues at check-in for a Miami to London flight, while another user posted pictures of blank screens.
“@British_Airways can you please just give your customers an update?” user Tessa Hartmann said on Twitter.
BA was criticised earlier this week for a similar disruption.
It was also hit by a major computer system failure in 2017 that stranded 75,000 passengers over a holiday weekend, sparking a public relations disaster and pledges from the airline that it would do better in future.
The company said: “We're investigating and working hard to resolve a technical issue and apologise for the inconvenience.
“We will provide further updates as quickly as possible.”