Thirty jobs are at risk at Irish airline Aer Lingus as it considers permanently axing its Belfast to Heathrow due to Brexit issues.
The airline said if the decision is implemented on 26 March, it would lead to the closure of its Belfast base.
Belfast staff were told on Wednesday, with the possibility of redundancy or transfer to other bases discussed.
Aer Lingus Regional flights, which are operated by Emerald Airlines would not be affected, Aer Lingus has said.
Emerald Airlines took over Aer Lingus' regional routes after Stobart Air collapsed in 2021.
Passengers booked on the Belfast City Airport to Heathrow route up until 25 March will be unaffected if the closure proceeds.
Anyone with tickets for the route after that date will be contacted by Aer Lingus before the end of January.
In a statement, the airline said: "As an EU carrier, regulatory changes arising from the withdrawal of the UK from the EU, have had an impact on Aer Lingus' traffic rights as a carrier within the UK market and, in particular, on the operation by Aer Lingus of the Belfast-London Heathrow route.
"Despite constructive discussions with the relevant authorities in the UK, we have, to date, been unable to identify a viable solution.
"We have therefore advised our Belfast-based staff of the likelihood that we will have to cease our Belfast-London Heathrow operation (which we have operated for 15 years), with effect from 26 March 2023, leading to the closure of our Belfast base."
Because of the UK traffic rights issue, British Airways has been carrying passengers on Aer Lingus's Belfast-Heathrow for the past few weeks.
Passengers have been booking with Aer Lingus but then travelling on BA planes, according to a spokeswoman for Belfast City Airport.
BA runs the same number of flights at exactly the same times, so "there has been no disruption to the schedule" on the airport's Heathrow route she said.
The spokeswoman added that she hoped BA would continue to operate the route if the Aer Lingus closure goes ahead.
BBC News NI has contacted BA and is awaiting a response.
Regarding the Belfast jobs, Aer Lingus said with it was consulting with union representatives and employees to "make available as many options as possible in the event of the closure of our Belfast base".
It said those option include "transfers to another base, so as to minimise the potential need to make a number of redundancies among our Belfast based staff".
The airline advised passengers it will honour all bookings on its Belfast-London Heathrow service "up to and including 25 March 2023".
"In the event of the route being closed, those customers with flight bookings beyond 25 March 2023 will be contacted by Aer Lingus by the end of January 2023, to provide alternative flight arrangements or a refund," the statement said.
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