United is to axe its Belfast International–New York service from January, following a decision by the European Commission to block funding that could have saved the route.
The Northern Ireland Assembly had pledged £9 million to the US airline after United confirmed the route would be cut because of “poor financial performance”.
The last departure from Newark/New York will be on January 9.
Belfast International said the ruling by the European Commission to block the funding package to protect Northern Ireland’s only scheduled trans-Atlantic service “defies logic” and is an example of “abysmal Brussels decision-making”.
“You could hardly get a worse example of process-driven madness,” said airport’s managing director Graham Keddie. “To block a support package for an airline that delivers direct access to the United States is almost beyond comprehension.”
“This is a vital link for business and losing it will be a body blow to executive ministers who use it to promote Northern Ireland to would-be investors from the United States.”
Keddie also warned the decision could put jobs at risk.
United said it would refund the initial payment from the Northern Ireland government and not accept any further payments.
Keddie described the decision as “bizarre” and “no way competes or conflicts” with competitors within the United Kingdom. “The EU decision-making process is abysmal, biased and unfair and has resulted in the loss of this service.”
In a statement United said: “We have regretfully taken this decision because of the route’s poor financial performance. We will contact customers with bookings for flights beyond those dates to provide refunds and re-accommodate where possible. We apologise for any inconvenience caused.”
Ulster Unionist economy spokesman Steve Aiken told The BBC the withdrawal of the service was an "international embarrassment".
"We need to know what checks were made with the European Union over state aid and who took the decision that it was not an issue?" he said.
"This is a huge blow to our international standing and the minister must tell us what he intends to do to sort it out."