British Airways has apologised after IT issues caused cancellations and delays in and out of Heathrow Airport.
Passengers reported long queues at Terminal Five of the west London airport on Wednesday evening, with others describing the situation as "chaos".
BA said it had experienced a "systems issue", resulting in a backlog of flights and some cancellations.
It came just hours after a fire alarm at Heathrow's air traffic control tower caused a separate set of delays for a number of airlines.
The fire alert briefly forced flights to be diverted to other airports.
One passenger, John Hueston, complained of long delays at Heathrow and a "nightmare service".
"Service at Heathrow a nightmare. Over two hours in a queue to be moved to another queue," he tweeted.
"No information for passengers or assistance to re-book flights. No hotels left, but Heathrow staff had time to take photographs of queues."
Another, Barnaby Black, complained of being shepherded from one queue to another.
"Been sent from one very, very long queue to another very, very, very, very, very, very long queue," he tweeted.
"Please someone send help. Send food parcels."
A British Airways spokesperson said it was working hard to resolve the technical issue.
"As with a number of airlines we are experiencing some disruption at Heathrow a result of an issue with a supplier IT system," the spokesperson said.
"We are working with our supplier to resolve the matter and are sorry for the disruption to our customers’ travel plans."
Following the earlier fire alarm, Heathrow Airport said the operation of the airfield had been impacted for a "short while", resulting in some delays to different airlines.
A Heathrow spokesperson said: "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."
The diverted flights included one from Glasgow and another from St Petersburg.