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 5 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.
On Thursday morning, a BA spokesperson said: "Our flights are operating today, however we expect some knock on disruption from yesterday's combination of a temporary closure of Heathrow Airport's air traffic control tower and a supplier system issue which affected a number of airlines.
"We are doing all we can to keep disruption to our services to a minimum, and are sorry for the disruption to our customers' travel plans."
On Wednesday, 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".
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.