British Airways has apologised to passengers for delays after an IT glitch hit check-in systems.
Angry travellers complained of hours queuing at airports but the airline said it is not a worldwide problem as some passengers had described it.
A spokeswoman confirmed passengers are being checked in at Heathrow and Gatwick but said the process would be "a bit slower than usual".
Asked where the IT problems are, the spokeswoman said: "It is patchy."
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."
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."
Staff with clipboards were writing manual boarding passes for passengers, one delayed traveller at Seattle Airport said.
Matthew Walker had been waiting for more than two hours to board his flight back to Heathrow.
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."
In July the airline had to apologise after a glitch in its new check-in system caused delays.
BA began installing the new system at airports across the world in October last year and the rollout was completed earlier this year.
On the latest issues the airline said in a statement: "We are checking in customers at Heathrow and Gatwick Airport this morning, although it is taking longer than usual.
"We would encourage customers to check in online before they reach the airport. We are sorry for the delay to their journeys."