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.
The problems mean parts of BA’s website are unavailable and some travellers claimed they could not check in on the mobile app.
BA aircraft landing at Heathrow are unable to park up as outbound aircraft cannot vacate the gates, which has resulted in passengers being stuck on aircraft.
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”.
Heathrow advised passengers to check the status of their flight before travelling to the airport.
BA staff in Heathrow’s Terminal 5 were resorting to using white boards, according to passenger Gareth Wharton.
Delays have also been reported in Rome, Prague, Stockholm and Malaga 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.
The BBC’s Phillip Norton is at Rome International airport, where he has been waiting to fly to London.
He said BA staff were unable to say how long delays would be, telling him “all flights are grounded around the world”.
Alma Saffari is in Marseille waiting to get her flight back to Heathrow.
She said: “When we finally boarded the captain came out and told us their computer systems were down worldwide.
“Eventually after sitting on the tarmac for 1 and a half hours we disembarked the plane.
“Now we are sitting in the departure area outside the gate.”
Ms Saffari, who is with her 13-month-old baby, said she had been given a voucher for food and drink.
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