British Airways has apologised after flights due to depart from the US were grounded for several hours.
The airline said a technical issue that affected some of its long-haul flight planning systems overnight had now been resolved.
BA passengers reported waiting for several hours in airports, with flights eventually departing after long delays.
A spokesman for the airline said the company was "sorry for the disruption caused to our customers' travel plans".
It is understood that short-haul flights were not affected by the technical problems and there were no safety concerns.
There were long delays on long-haul flights, including those between London's Heathrow airport and the US and Canada, and other flights serving Gatwick airport, including arrivals from Cancun, Mexico.
Flights at Heathrow impacted by the delays are now operating, but Terminal 5 is expected to be busy this afternoon as a result.
"We have additional colleagues on hand to support passengers and get them on their journeys as quickly as possible," a spokesperson for the airport said.
It is not the first time BA has been hit by technical problems.
In February, the airline apologised after cancelling all short-haul flights from Heathrow due to IT issues.
On Monday night passengers took to Twitter to report waiting at John F Kennedy International Airport for more than three hours.
Others said they had been sitting on planes parked on runways for hours, before being moved back to the airport.
One of those delayed was British actress and model Elizabeth Hurley, who tweeted on Tuesday morning: "@British_Airways Stranded at Antigua airport with no food or water, taxis or hotels offered yet. Plane delayed 20 hours."
An hour later she said: "Still stranded – no food, water or hotel. Pretty dodgy service @British_Airways."
In a statement in the early hours of Tuesday, BA said: "Our flights due to depart the USA tonight are currently delayed due to a technical issue with our third-party flight planning supplier, which we are urgently investigating."
It confirmed hours later that the problem had been fixed.
Separately, the airline has been dealing with delays resulting from the cold weather last week, as some flights were held up due to planes needing to be de-iced.
This has had a knock-on effect on baggage handling, with people on social media sharing pictures of bags piled up at Heathrow.
"We're doing everything we can to reunite our customers with their delayed baggage as soon as possible," BA said.
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