Holidaymakers travelling with British Airways are facing delays and problems checking in online, with the airline saying system failures are to blame.
Customers at some UK airports have taken to social media to complain about long queues, flight delays and the app crashing.
British Airways said it is reverting to manual systems for check in at airports.
It said customers should allow extra time at airports.
In a statement the airline said: “We are experiencing some systems problems this morning which are affecting check-in and flight departures.
“Please check ba.com and manage my booking for the latest flight information and allow extra time at the airport.”
It said the issue was not a global outage, but a problem with two separate systems – one which deals with online check in, the other that deals with flight departures.
The problems are affecting flights across its network but not at every airport, it added.