British Airways has apologised to passengers for delays after an IT glitch hit check-in systems.
Angry travellers complained of hours queuing at airports and some reported they had been told the problem was worldwide.
Responding to passengers on Twitter, the airline wrote: "We apologise to our customers for the delay and we appreciate their patience as our IT teams work to resolve this issue."
It added: "Our colleagues are doing everything possible to check in customers for their journey."
One passenger stranded at Toronto Airport said: "I guess the check in application is down and it's world wide."
Ewan Crawford, of Glasgow, said he was at Chicago O'Hare International Airport.
He tweeted: "Never a good sign when they deliver water to the gate! Waiting at ORD for @British_Airways 296. Worldwide computer outage apparently! Hmm."