Computer problems are causing delays for British Airways passengers worldwide, the airline has said.
The carrier apologised for the “global system outage” and said it was “working to resolve the the problem as quickly as possible”.
Heathrow Airport said it was “working closely” with BA to solve the issue.
It is not known how many flights are affected but passengers have reported issues with flights to and from Heathrow, Gatwick and Belfast.
Latest updates on BA flight disruption
The problems mean parts of BA’s website are unavailable and some travellers claimed they could not check in on the mobile app.
Journalist Martyn Kent said he had been sitting on a plane at Heathrow for 90 minutes. He said the captain told passengers the IT problems were “catastrophic”.
BA staff in Terminal 5 were resorting to using white boards, according to passenger Gareth Wharton.
Delays have also been reported in Rome and Malaga airports due to the system failure.
Philip Bloom said he had been waiting on board a Heathrow-bound flight at Belfast for two hours.
He added: “We haven’t been told very much just that there is a worldwide computer system failure.
“We were told that we couldn’t even get on other flights because they are unable to see what flights we can be moved to.”
BA flights at Gatwick airport are also grounded, according to travel journalist Phil Davies.
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