British Airways passengers have been hit by long delays after an IT glitch affected worldwide check-in systems.
Angry travellers were forced queued for hours, as airport staff manually processed flight checks-ins. Some passengers posted photographs on social media of hand-written boarding passes.
British Airways told ITV News the glitch has affected "a number of airports" around the world but it is unclear how many.
"Our IT teams are working as hard as they can to quickly fix a problem with our check-in system," a spokeswoman said.
She added: "We are sorry for the delays some customers are experiencing as they check-in for their flights."
Jack Maddox, from London, was due to be flying from Atlanta in the US to Heathrow on Monday night.
Speaking to ITV News from the terminal building, he said: "It was at first meant to be a five minute wait but it's now been four hours since their computer systems went down.
"A customer service manager said they had it up and running a while ago but it went right back down.
"Now they're processing manually, hand writing labels of the remaining 100 or so passengers on my flight. It was meant to take off an hour and 40 minutes ago.
"We're not even close to boarding. The plane is sitting fully fuelled and ready to go. There are a lot of agitated people with connecting flights."
It is not yet clear whether any BA flights have been cancelled.