Stansted said passengers arriving on the Lisbon flight at 9.20am had to be processed by departures security before moving to arrivals, causing delays.
Ryanair said its crew had disembarked passengers “to the assigned gate”.
An investigation into the incident is being conducted by the airport, the BBC reported.
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 Ryanair spokesman said: “The crew of this flight correctly directed disembarking customers to the assigned gate.
“However, customers 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.”
Unhappy passengers took to Twitter to express their frustrations, with some complaining of queues of two hours.
Sourced from Travel Weekly