Delta Air Lines has resumed "limited" flights after a "major system-wide network outage" saw its jets grounded.
Air travellers faced long queues to check in at Heathrow on Monday after a power outage in Atlanta at 2.38am local time plunged the airline's operations worldwide and computer systems into chaos.
The company, which operates flights out of Edinburgh, Heathrow and Manchester, has since said the "ground stop has been lifted and limited departures are resuming" but that "delays and cancellations continue".
A spokeswoman added: "Customers heading to the airport should expect delays and cancellations. While inquiries are high and wait times are long, our customer service agents are doing everything they can to assist."
Passengers were left stranded as flights across the world were affected by what the company said on Twitter was a "major system-wide network outage". Delta said it operated just 800 of its nearly 6,000 scheduled flights.
"We apologise to customers who are affected by this issue, and our teams are working to resolve the problem as quickly as possible," the spokeswoman added.
Peter Taylor from Coventry had been due to fly to the American city of Boston on the 9.40am Delta flight from Heathrow's terminal three.
The 58-year-old told the Press Association he was informed by staff that they were "having some system problems" and had to "manually" give them his pre-boarding details to write down.
Mr Taylor said he and his fellow passengers were kept in the lounge and that it became clear they were not going to be taking off on time.
"We were given the update that this was pretty serious and that it could be anything from an hour and six hours," he added of the delay.
He described the feeling amongst the travellers as "classic British shrugging of the shoulders and getting on with it".
"There was a lot of handwritten notes and bits of paper around - which is very bizarre to see in this modern age," he added.
Having given up on the long wait, sticking out five hours before he quit, Mr Taylor said: "The result for me is that I am going to have to spend a couple of long afternoons and evenings to remotely attend meetings that I really needed to attend face to face."
Frustrated passengers also took to social media to voice their outrage at the delays - with some reporting handwritten tickets being used by the airline.
Amanda Jackson said on Twitter: "Chaos trying to check in @Delta Heathrow t3. Been in queue for 1.5 hours. You seriously need to open more desks to overcome technical hitch."
Cassie Chou took to Instagram to share a picture of inside the airport and added a caption saying Delta staff in Heathrow were "handwriting tickets manually".
The airline began in 1924 as an aerial crop-dusting operation called Huff Daland Dusters, and now serves more than 160 million travellers, according to its website.