Hundreds of passengers have been queuing for several hours at Manchester airport due to a “technical failure” with the check-in system.
A total of 12 Thomas Cook flights were affected after staff had to switch to a manual check-in system, leading to delays of about two hours.
Two flights at East Midlands airport were also affected, the airline said.
Passengers were being advised to turn up in advance as planned, the BBC reported.
A Thomas Cook spokesman said: “We’re very sorry for the frustration caused to our customers at Manchester Airport check-in this morning.
“Our system is back up and running and we’re working hard to catch up and get our customers on holiday as soon as possible.”
There were delays to flights of around two hours, according to the operator.
Manchester airport earlier apologised and said: “Due to a technical issue this morning several airlines are having to check in passengers manually, which can take longer than normal.”
Alex Neill, Which? managing director of home products and services, said: “Passengers who have faced a severe delay to their flight as a result of today’s disruption may be entitled to compensation. Airlines should proactively tell them if they can claim.
“It’s time for airlines to start automatically compensating eligible passengers. Until they do this, we urge people to check what they are entitled to.”
More on flight delays:
Toilet roll shortage delays British Airways flight for five hours