British Airways has apologised after IT issues caused dozens of flights in and out of Heathrow Airport to be cancelled, described by one passenger as "complete chaos".
Other flights were delayed as passengers reported long queues at the London airport's Terminal Five on Wednesday evening.
One traveller said: "They just said to me 'stay in the queue'. It's not terribly helpful…It's complete chaos."
Another said: "We're really not getting any information at the moment. Nobody's being sorted out…They can't re-book the flights because they've got no computer system. Everyone's just sitting here."
Hundreds of passengers were told to return on Thursday.
The airline said it had experienced a "systems issue", leading to a backlog of flights and some cancellations.
BA tweeted: "We are experiencing disruption to our flights as a result of an issue with some of our IT systems. We are working hard to urgently resolve the matter and are sorry for the disruption to some customers' travel plans."
We are experiencing disruption to our flights as a result of an issue with some of our IT systems. We are working hard to urgently resolve the matter and are sorry for the disruption to some customers travel plans.
— British Airways (@British_Airways) July 18, 2018
Earlier, a number of airlines suffered a separate set of delays caused by a fire alarm at Heathrow's air traffic control tower, forcing some flights to be diverted to other airports.
The alert affected BA services at Terminal 5.
A Heathrow Airport spokesman said the alarm "impacted operation of the airfield for a short while" on Wednesday afternoon.
"As a result of the temporary closure of Heathrow Airport's air traffic control tower earlier this afternoon, a small number of flights from all airlines were forced to divert to alternative airports," he added.
"We are doing all we can to keep any knock-on disruption to our services to a minimum and are sorry for the disruption to our customers' travel plans," he said.
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A Gatwick Airport spokesman said: "We only had two diverts from Heathrow Airport as a result of the earlier action – one from Glasgow at 5pm and the other was from St Petersburgh at 5.10pm."
The disruption comes as airports across the country gear up for the busiest part of the year as the summer holiday season begins in earnest.