Air travellers have been warned to check the status of their flights after Delta Airlines grounded its outbound jets.
A computer outage has been blamed by the company for the chaos which they say has "impacted flights". The firm operates out of Heathrow, Manchester and Edinburgh airports.
On Monday morning flights across the world were affected by what the company said on Twitter was a "major system-wide network outage".
A spokeswoman told the Press Association: "Delta experienced a computer outage that has impacted flights scheduled for this morning.
"Flights awaiting departure are currently delayed. Flights en route are operating normally.
"Delta is advising travellers to check the status of their flights this morning while the issue is being addressed."
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.
Frustrated passengers have taken 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."
Peter Taylor added: "Still in Heathrow - should have been on 9:40 flight to Boston - waiting for news - staff apologetic, even had the captain speak to us."
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".
A Heathrow spokesman said: "Delta flights from Heathrow are experiencing delays due to the worldwide technical issue with their computer systems.
"Check-in is currently operating using a backup system and airport staff are on hand to assist any passengers that are impacted by the delays.
"Passengers should check with the airline for updates on their flights."
A Virgin Atlantic spokeswoman said their flights are departing as normal, but that some of their customers may be "booked on a codeshare ticket" with their partner, Delta.
She added: "If so, customers should check the status of their flights, whilst Delta addresses the computer outage that has impacted its flights scheduled for this morning."
Delta said a "power outage" in Atlanta at 2.38am local time impacted its computer systems and operations worldwide - resulting in flight delays and cancellations.
A spokeswoman said the company's technology team is working to resolve the problem - with flight status information displayed on airport screens, delta.com and some mobile and airport technology currently affected.
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