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.
On Twitter, one holidaymaker at Gatwick Airport said he was stuck on the ground because the "pilot can't get data".
Another disgruntled customer at Manchester Airport said he was meant to be on the BA1385 flight, but was told at the gate that there was "worldwide British Airways system outage".
British Airways said it is working as quickly as possible to resolve the systems issue which "has resulted in some short-haul cancellations and delays from London airports."
It said: "We are offering customers booked on short-haul services departing from Heathrow, Gatwick and London City today, the opportunity to rebook to another day."
Source: BBC