Budget airline bmibaby is pulling out of Belfast next month, spelling the end of its links to 11 destinations in England and Europe.
The UK carrier, recently acquired by British Airways parent International Airlines Group, said it was consulting with all staff on options including a complete closedown in September.
However, its flights from George Best Belfast City Airport to eight continental destinations, as well as Birmingham, East Midlands and London Stansted airports, will stop on June 11.
The airline does not employ Belfast-based crew and uses ground handling agents for check-in and customer service.
City Airport chief executive Brian Ambrose said it had been assured by the airline that customers would be offered an alternative travel date or a full refund.
He urged passengers booked to travel after June 10 to contact bmibaby on 0844 245 0055.
Mr Ambrose added: "Like thousands of passengers, we are very disappointed at this decision.
"Strong advanced bookings proved there was significant demand for the sunshine routes operated by bmibaby.
"However, I am confident that we can attract other airlines to the fill the routes vacated by bmibaby."
IAG was given a substantial discount on the £172.5m price-tag for BMI, coveted for its Heathrow landing slots, when it reluctantly acquired bmibaby and bmiregional as part of the deal, finalised last month. Former BMI owner Lufthansa had tried to off-load both smaller airlines to no avail.
Peter Simpson, BMI interim managing director, said: "We recognise that these are unsettling times for bmibaby employees, who have worked tirelessly during a long period of uncertainty.
"Bmibaby has delivered high levels of operational performance and customer service, but has continued to struggle financially, losing more than £100m in the last four years. In the consultation process, we will need to be realistic about our options.
"To help stem losses as quickly as possible and as a preliminary measure, we will be making reductions to bmibaby's flying programme from June.
"We sincerely apologise to all customers affected and will be providing full refunds and doing all we can with other airlines to mitigate the impact."
While bmibaby services from Belfast are stopping, the flight operated by sister airline BMI to Heathrow from Belfast is continuing.
Ulster Unionist MLA Michael Copeland said bmibaby's decision would inconvenience "thousands of people".
"To announce the closure of routes with just over five weeks notice is simply not acceptable.
"It's all very well for BMI to say this will not affect the mainline BMI service from Belfast City to London Heathrow but people will understandably be very concerned about its future too."
Meanwhile, rival budget airline Ryanair, which pulled out of Belfast City in 2010 after a bitter row over extending its runway, said it would "rescue" the plans of bmibaby passengers flying from Belfast and Birmingham.
The Irish carrier said it was offering fares from just £29.99 (one way) to corresponding destinations on Ryanair flights to/from Derry, Dublin and East Midlands airports.
Bmibaby operated 11 routes from Belfast City: Birmingham, East Midlands, London Stansted, Amsterdam, Geneva, Malaga, Alicante, Palma, Faro, Ibiza, Menorca. Out of the 2.4m passengers who passed through Belfast City Airport in 2012, 420,000 flew with bmibaby. Bmibaby was launched in 2002.