Aer Lingus is stopping flights from Belfast City Airport to several UK destinations but is offering some passengers a replacement flight.
It comes after Stobart Air, which operates Aer Lingus' regional services, including those from Belfast, failed to find a buyer and is being liquidated.
Stobart Air also operated a number of routes within the Republic of Ireland for Aer Lingus and flights on those routes have all been cancelled.
Aer Lingus was due to replace Stobart Air with a new regional operator next year but it is not clear how quickly it could now start to operate the services.
Emerald Airlines was being lined up to take over the contract.