Thousands of British Airways passengers were forced to wait for several hours to check in at Heathrow Airport due to a fault with the baggage system.
The issues began at about 08:00 BST with staff having to check in luggage using a slower manual procedure.
Terminal 5 was the worst affected, although the fault also occurred at Terminal 3 and Gatwick Airport.
British Airways (BA) said the system was now up and running and the backlog should be cleared “soon”.
The airline said a “small number of flights left without all customers’ baggage loaded”, with luggage being “put on the next available flight”.
Comedian Eddie Izzard was among those caught up in the problem, tweeting that passengers had been “waiting a long time”.
Other travellers complained that they had missed flights as a result.
Passenger Andy Gibbs flew to Bologna Airport in Italy with his wife and children but said that their luggage was missing.
He told the BBC: “I’m standing in a queue with all of the rest of the passengers and none of them have bags and they’ve all been told to fill in a lost luggage form.
“Then the text that has come through from BA has said we’ve experienced problems and there’s a good chance that your luggage won’t have made it on to that flight. No-one in our queue here has their luggage.”
Heathrow Airport said the problem “has now been resolved following a fault” and it was “working closely with BA to get check-in running smoothly again”.