Irish airline Aer Lingus will suspend its services between Belfast City airport and Heathrow at the end of the month.
Sister airline, British Airways, will now operate the three-flights-a-day service over the winter months.
Dublin-based trade union Fórsa blamed Brexit for the move because the airline’s Air Operator Certificate (AOC) expires at the end of October.
As an Irish carrier, Aer Lingus cannot operate scheduled services on UK domestic routes.
A UK Civil Aviation Authority spokesperson said: “After the UK left the European Union, all UK-based airlines have to operate under a UK operating licence which governs the traffic rights they may hold.
“Under current legislation and international agreement, EU carriers such as Aer Lingus, are not permitted to operate scheduled services on UK domestic routes.
“The current service operated by Aer Lingus UK between Belfast and London currently utilises a wet-leased capacity arrangement from its parent company, the Irish airline, Aer Lingus.
“This has been operating in this arrangement for some 24 months whilst a long-term solution was sought.
“We’ve been flexible with Aer Lingus to find a sustainable solution which satisfies both the requirements of the UK-EU Trade and Cooperation Agreement that no longer relies on the wet-leasing arrangements.”
An Aer Lingus spokesperson said: “Aer Lingus is very keen to continue operating the Belfast City-Heathrow London service, which we have been operating since 2007.
“We are engaging with the relevant authorities in order to allow us to continue to serve this route into the future.
“For the upcoming winter season, we will be working with our sister airline, British Airways, to ensure there is continuity of service and no impact to any of our passengers’ journeys.”
Other flights under the brand from Heathrow are operated by Emerald Airlines, which is Aer Lingus’ franchise partner.
A Fórsa official said that the loss of the Belfast City-Heathrow route will be “hugely disruptive” for the Aer Lingus cabin crew in Belfast who will be redeployed.
“This is, unfortunately, another unforeseen and disastrous consequence of Brexit,” he said.
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