Belfast International Airport has apologised to passengers for delays at security, describing the situation as "an unacceptable standard of service".
The latest of what have been recurring problems happened on Monday morning.
Some passengers told BBC News NI they waited about one hour to get through security to their departure gates.
The airport's security contractor, ICTS, is currently training an extra 40 new staff following a recent recruitment drive.
A spokesman for the airport said "additional support staff" were also being drafted in from elsewhere.
Passengers have been advised to allow "at least two hours" to get through the airport.
One passenger, Adrian Huston, said people in queues on Monday were panicking they would miss flights.
"There must have been 1,000 people waiting to be processed.
"All the lanes were staffed and open but they could not seem to deal with the volume of passengers.
"They had to turn off escalators to stop the queue falling back on itself."
The airport has seen passenger numbers jump to record levels over the past year, largely due to the arrival of Ryanair.