The British government has cut taxes on long-haul flights from Northern Ireland to help protect routes against competition from airports in the Republic where duties are lower.
Air Passenger Duty will fall to the lower short-haul rate, which is £12 for economy and £24 for business and first-class flights, with effect from November 1st.
The current rate for a flight from Belfast to the US is £60 for economy and £120 for business and first class, against only €3 on similar flights from Dublin.
The move comes after Continental Airlines warned that Northern Ireland's only transatlantic route - which is operates between Belfast and Newark - could be axed if the duty was not cut.
"The government has taken proactive measures to protect the only direct long-haul service operating from Northern Ireland and with it the jobs of those who serve the Belfast route," finance minister George Osborne said. "Northern Ireland faces a unique challenge in attracting traffic - including very valuable business customers - into its airports."
The British government said it planned to devolve responsibility for aspects of the tax to the Northern Ireland Assembly.