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 at a number of airports on social media.
Journalist Martyn Kent said he had been sat 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.
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.”
By: BBC News