British Airways issued an apology after passengers faced long delays at check-in at Heathrow and Gatwick on Sunday due to a systems glitch.
The airline is upgrading its check-in system and problems led to lengthy queues on one of the busiest days of the year for the airports.
Travellers, including TV presenter Philip Schofield, took to social media to voice their anger. Schofield ended up missing his flight from Heathrow.
BA apologised for the delays and said it had extra staff in place to help customers. Some were rebooked on alternative flights, it said.
The airline told the BBC it had been phasing in the introduction of a new world-wide check-in system since October 2015, which more than 10 million customers have already used.
A spokesman added: “Our IT teams worked very hard to quickly fix a short-lived problem with our check-in system. We are sorry for the delays some customers had as they checked in for their flights.”