Two airlines are cutting routes at both Belfast International and Belfast City airports.
Ryanair is understood to have reduced a number of services at Aldergrove this winter.
Meanwhile, Aer Lingus has confirmed it will no longer be offering two routes at Belfast City Airport from next summer.
The Belfast airports have both expressed their disappointment at the flight cuts.
From November, it will no longer be possible to book Ryanair flights between Belfast and Berlin, Manchester and Lanzarote.
The BBC has seen emails sent from Ryanair to customers cancelling flights to Manchester in which it said “Ryanair regrets that following a commercial review” certain flights will cease operation from 6 November.
When it announced its winter schedule, the carrier said these routes would continue to be operated.
In March, Ryanair said it was dropping routes to Poland and Malta from Belfast International, while also reducing the number of flights it operates to London Stansted and Manchester.
The carrier said this was due to the “weak UK market” and UK Air Passenger Duty (APD).
APD is a tax levied on air passengers, which varies according to destination and class of travel.
A spokesperson for Belfast International said it was “extremely disappointed” at the further reduction in service.
The airport said it again highlights the disadvantage posed by APD, which it said is “denying Northern Ireland passengers destination choice”.
“There remains a strong market demand for direct air services to those destinations which will no longer be served by Ryanair,” the spokesperson said.
“We have already secured some additional capacity to the Canary Islands and will continue to work with our airline partners to replace lost services.”
In July, Ryanair said a number of loss-making bases will be cut or closed this winter due to the late delivery of the Boeing MAX aircraft.
When contacted about the route reductions at Belfast International, Ryanair said: “As announced on 16 July, due to the late delivery of up to 30 Boeing MAX aircraft, a number of loss-making Ryanair bases will be cut or closed this winter.
“We are working hard to minimise the impact on our customers and our people, and to continue as many routes as possible, which will be served by flights from other bases.”
But it declined to comment specifically on Belfast.
Meanwhile, Aer Lingus said it will no longer operate services from Belfast City to Faro and Malaga during summer 2020 following a “commercial review”.
“Aer Lingus remains committed to Belfast City and continues to operate a high frequency daily service between Belfast City and London Heathrow.”
The airport said it is disappointed at the decision.
“While this news will come as a disappointment to the thousands of passengers that filled the Faro and Malaga flights during the summer months, the airport will continue to work closely with Aer Lingus and IAG to explore other international routes.”