British Airways planes have been grounded following a computer fault.
Major disruption to the flag carrier airline's schedules was reported early on Tuesday following the IT error, which left planes unable to access the global flight plan system.
The issues appear to have primarily affected inbound planes flying to London from North America, according to reports.
British Airways said short-haul flights and those that have already departed have not been affected,
A tweet from the airline to a customer described a "technical issue with our third-party flight planning supplier", adding that they were "working hard to resolve this as quickly as possible".
Ben Richardson, a managing partner at health systems firm Carnall Farrar, said on Twitter that his flight from Toronto had not departed as the amount of time allotted for its crew to take off had timed out.
He wrote: "Currently in Toronto and BA092 just timed out. Matter of time until every BA plane in the Americas times out I’m afraid. Crew says it’s a German flight control system."
Mr Richardson also said that bottlenecks had appeared to form in the airport as a result of passengers having to leave their planes.
Any passengers with a place to stay in the area were advised to do so, he added.
A statement from British Airways said: “We’re experiencing delays to some of our flights due to a technical issue with our flight planning. We're sorry for the disruption to our customers' journey, we’re urgently investigating this so that they can travel as soon as possible.”
The new disruption comes only 24 hours after frustrated passengers at Heathrow Airport claimed they had to leave without their Christmas presents after their luggage failed to arrive following long waits at baggage reclaim.
The delays were branded by angry customers as an "absolute shambles", with many of the impacted customers appearing to have been travelling with British Airways.
One frustrated passenger tweeted: "@HeathrowAirport is in absolute shambles. No ground staff available to dock the plane so passengers (including us) are sitting for hours on the plane after landing. No baggage handlers either. Hundreds of people waiting for their suitcases for hours. What on earth is going on?"
Another said: "@British_Airways @HeathrowAirport after 3 hours of waiting for our suitcases at baggage reclaim we have now been told to go home and submit a lost luggage report while our bags are still sat on the plane. Absolute shambles."
A statement from Heathrow Airport said that it is the duty of the airline to ensure luggage is returned to its owners.
