British Airways has apologised to passengers for delays after an IT glitch hit check-in systems.
Angry travellers complained of hours queuing at airports in Europe and the US, while some passengers also experienced problems with online check-in.
Responding to passengers on Twitter in the early hours of Tuesday morning, 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."
A BA spokeswoman later said the airline was "checking in normally across all of our airports", but added that the process at Heathrow and Gatwick would be "a bit slower than usual".
Ewan Crawford, of Glasgow, who was waiting at Chicago O'Hare International Airport for a flight to Heathrow, tweeted: "Never a good sign when they deliver water to the gate! Waiting at ORD for @British_Airways 296. Worldwide computer outage apparently! Hmm."
Never a good sign when they deliver water to the gate! Waiting at ORD for @British_Airways 296. Worldwide computer outage apparently! Hmm.— Ewan Crawford (@ewancrawford) September 6, 2016
Matthew Walker, another passenger hoping to fly from the US to London, said he had been waiting for more than two hours to board his flight at Seattle Airport.
The 29-year-old financial analyst, who lives in London but is originally from Australia, checked in online before arriving to catch his flight but said staff on the ground could not access their computer systems to see which passengers had gone through security.
Speaking from the airport, he told the Press Association: "People were lining up, some had already checked in and got through security, but others, when this thing happened, whatever it is, were stuck in the check-in queue.
"So they (the staff) have the problem that they didn't know who had already gone through the gate because all the systems literally just had a meltdown, basically."
Magazine writer Stefano Andrean said: "I've waited an hour so far at Berlin airport check-in for Heathrow. No information from staff at all. Not acceptable."
Chris Black, from London, described the BA check-in process in Polish capital Warsaw as "rubbish".
Passengers at Heathrow Terminal 5 reported waits of about 45 minutes to check in.
Elaine and Paul Barnett, who had come from Sheffield to travel to Sardinia, said the process had taken "longer than usual" and they had been required to give extra details once they reached the desk.
"You really have to get here early and expect that it's busy," Ms Barnett said.
Earlier problems appeared to have been smoothed out by early morning, with most queues moving.
Patrick Darby, from Dulwich, south London, who was travelling to Russia, said: "There was a hold-up when nothing seemed to happen but that has eased up now."
In July BA had to apologise after a glitch in its new check-in system caused delays.
The airline began installing the system at airports across the world in October last year and the rollout was completed earlier this year.
On the latest issues, BA said in a statement: "We are checking in customers at Heathrow and Gatwick Airport this morning as normal, although it may take longer than usual.
"We would encourage customers to check in online before they reach the airport."