There have been long delays at Belfast International Airport which saw passengers queuing outside the terminal and missing flights.
Travellers have posted photographs on social media, showing the extent of the queues.
Last month, the airport said it was increasing the lanes to eight.
However, the airport said it the number of lanes would have to be reduced to allow the work to be carried out.
Graham Keddie, managing director of the airport, has apologised, adding that work on the lanes would be finished before Easter.
Around 70 people missed their flights on Monday, Mr Keddie told Talkback on Radio Ulster.
Some scenes here at Belfast International Airport @BelfastAirport. The queue for security goes out the door of the terminal!!!! @QuintinOliver pic.twitter.com/wxkKA9rmKN
— Brandon Hamber (@BrandonHamber) March 11, 2019
Sinn Féin MLA Declan Kearney has called on management to provide firm reassurance to passengers that journeys will not be adversely affected by security delays.
The South Antrim MLA said prior to this, there has been a long standing and unacceptable pattern of security delays for passengers when travelling through the International airport.
“This leads to passenger frustration and anxiety over making flights.
“However, this morning queues for security were so long that passengers had to wait outside the airport in freezing conditions. As a result, a number of passengers did indeed miss their flights.
“There is now widespread uncertainty about whether Belfast International Airport security systems can guarantee passengers will be processed on time. That has implications for airport services and overall commercial reputation.
“Belfast International Airport management as a matter of urgency needs to provide firm reassurance to passengers that airport systems and services are going to function efficiently from now on, and that there will be no repeat of the chaos which occurred this morning,” he added.
Missed an important flight to London this morning for work. Queues for security were out the door of the terminal and no updates or info available from Belfast International management. Not good enough @belfastairport pic.twitter.com/IaGa1SUWJY
— Beth (@Bethmollylou) March 11, 2019
Our security search area is expanding to 8 lanes! To do this we will reduce our security lanes from 6 to 5 lanes until Mid- March. We apologise for any inconvenience caused & continue to remind customers to arrive 2 hours before departure. Thank you for your ongoing patience. pic.twitter.com/T47BgW9a5R
— Belfast Airport (@belfastairport) March 8, 2019
@belfastairport @BelTel @belfastlive Belfast International security – absolute joke this morning pic.twitter.com/5dHSUlmz1Q
— Kelley McBride (@kelleymcbride) March 11, 2019
@belfastairport @easyJet Was here in plenty of time but not even in the airport yet. How will I make my 8.15 flight? pic.twitter.com/EyGDPTzWr7
— Paul Fitzpatrick (@pfitzp) March 11, 2019