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.
Why are you making commenting on The Herald only available to subscribers?
It should have been a safe space for informed debate, somewhere for readers to discuss issues around the biggest stories of the day, but all too often the below the line comments on most websites have become bogged down by off-topic discussions and abuse.
heraldscotland.com is tackling this problem by allowing only subscribers to comment.
We are doing this to improve the experience for our loyal readers and we believe it will reduce the ability of trolls and troublemakers, who occasionally find their way onto our site, to abuse our journalists and readers. We also hope it will help the comments section fulfil its promise as a part of Scotland's conversation with itself.
We are lucky at The Herald. We are read by an informed, educated readership who can add their knowledge and insights to our stories.
That is invaluable.
We are making the subscriber-only change to support our valued readers, who tell us they don't want the site cluttered up with irrelevant comments, untruths and abuse.
In the past, the journalist’s job was to collect and distribute information to the audience. Technology means that readers can shape a discussion. We look forward to hearing from you on heraldscotland.com
Comments & Moderation
Readers’ comments: You are personally liable for the content of any comments you upload to this website, so please act responsibly. We do not pre-moderate or monitor readers’ comments appearing on our websites, but we do post-moderate in response to complaints we receive or otherwise when a potential problem comes to our attention. You can make a complaint by using the ‘report this post’ link . We may then apply our discretion under the user terms to amend or delete comments.
Post moderation is undertaken full-time 9am-6pm on weekdays, and on a part-time basis outwith those hours.
Read the rules here