Thousands of passengers faced delays today after people arriving on a Ryanair flight went through an exit door "left open in error".
Passengers waiting in the departures lounge at Stansted Airport were forced to go through security again after a group arriving on a flight from Lisbon mistakenly entered the airport through the wrong door.
Stansted said passengers faced "long security queues" and delays of up to an hour on some flights, as it drafted in extra staff to deal with the incident.
A spokesman for the airport said an investigation was under way to determine who was at fault for leaving the door open.
A Ryanair spokesman said: "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 rescreened departing passengers. A number of flights were delayed throughout the day due to earlier issues with de-icing facilities."
Disgruntled passengers took to Twitter to express their frustrations, with some complaining of queues of two hours.
Rob Treloar tweeted: "Pandemonium at Stansted Airport. Security queue stretches length of terminal twice!"
Duncan McKay wrote: "Ongoing security 'issue' at Stansted Airport. Massive queues as no one can get through security. Communication over PA has not been great."
James Carter tweeted: "Massive security queues at Stansted, again. Been in line for two hours now."
Charlotte Carey added: "Whole of Stansted airport has to be re screened because of a security breach."
And Tom Wallace said: "Yet again, Europe's favourite 'airline' delivers. Shambolic, I think would be the word of choice."