Passengers travelling with budget airline Ryanair were made to disembark at a departure gate instead of arrivals, which led to delays of over an hour at UK's Stansted Airport.
Due to the blunder, thousands of passengers had to go through security queues twice.
The IB Times quoted airport staff as saying that the confusion occurred when passengers travelling to the UK on the Ryanair flight arriving from Lisbon walked through the wrong door, leading them to the departures lounge rather than the arrivals gate.
A spokesman for Stansted Airport said in a statement: "We are now experiencing queues in the security area with delays of up to an hour on some flights. We apologise for the inconvenience and hope to resolve this matter as soon as possible.
"We advise passengers travelling to the airport this afternoon to allow a little extra time for their journey."
The Mirror quoted a Ryanair spokeswoman as saying: "The crew of this flight correctly directed disembarking customers to the assigned gate, however they entered the airport through a door, which had been left open in error.
"Ryanair fully cooperated with Stansted Airport as it re-screened departing passengers. A number of flights were delayed throughout the day due to earlier issues with deicing facilities."
The incident happened on a busy day when thousands were returning after the Christmas break.
Officials have announced that an investigation will be launched to look into the error, reported The Mirror.
Image: Stansted Airport's terminal building near the check-in area. Photo: courtesy of Christopher Kern.