British Airways has warned passengers should expect “some knock-on disruption” following IT issues on Wednesday.
The airline apologised after dozens of flights in and out of Heathrow Airport were cancelled on Wednesday, with one passenger describing “complete chaos” at Terminal 5.
Long queues were reported as other BA flights suffered delays.
Hundreds of passengers were told to return to the airport on Thursday.
A BA spokesperson that “flights are operating today” and they are “doing all we can to keep disruption to our services to a minimum”.
The airline blamed a “combination” of problems – including a systems issue and the temporary closure of Heathrow’s air traffic control tower.
Numerous airlines were affected by a separate set of delays on Wednesday afternoon as a fire alarm was set off at Heathrow’s air traffic control tower, causing some flights to be diverted to other airports.
A Heathrow Airport spokesperson said the alarm “impacted operation of the airfield for a short while”.
One traveller caught in the disruption 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 rebook the flights because they’ve got no computer system. Everyone’s just sitting here.”
Some passengers expressed their frustrations on Twitter.
However, others appeared happier with the response of British Airways staff.