Dozens of British Airways flights into the UK have been delayed or cancelled after what the airline has described as a "technical issue".
Flights from the US, India and Japan were showing up as delayed.
"Our teams are working hard to resolve a technical issue which is affecting some of our flights," BA said on Twitter, in response to a passenger who had been delayed.
Some people have been put up in hotels and booked on other flights, it said.
A spokesperson for Gatwick Airport, where a number of BA flights have arrived late, blamed the delays on an issue with the airline's system for handling flight plans.
They said the problem caused delays of around three hours for some long-haul flights arriving into the airport, which had a knock-on effect on departures.
The worst affected was Flight BA170 from Pittsburgh, in the US, which was more than 12 hours behind schedule.
It is the latest technical issue to affect the airline, which faced massive disruption in August when more than 100 flights were cancelled because of an IT glitch.
In a statement, BA said it was "very sorry for the disruption".
It told customers to check its website for updates and make sure their contact details were up to date.
British Airways was hit by its first ever pilot strike in September when flight crews walked out in a row over pay and conditions.
Later that month, the airline revealed the industrial action had cost it at least €137m (£121m).