The US has ruled that Canada's Bombardier received government subsidies and sold C-Series jets below cost in the US, a step likely to lead to steep tariffs.
The US Commerce Department investigated the aerospace firm's US sales after a petition from rival US company Boeing.
The dispute has contributed to escalating trade tensions between the US and Canada.
The conflict also has the potential to lead to job losses in Northern Ireland.
The fight stems from a 2016 sale of 75 C-Series jets to Delta Air Lines. Boeing claims Delta paid $20m per plane, well below an estimated cost of $33m and what Bombardier charges in Canada.
Bombardier employs about 1,000 people in Belfast linked to the C-Series.
The Commerce Department's final determination on Wednesday set trade duties of about 292% – slightly lower than a preliminary finding.
The inquiry now moves to the US International Trade Commission, which will examine if the dumping and subsidies caused injury to Boeing. It is expected to make a final decision in February 2018, which would trigger the duties.
This week, Canada's ambassador to the US warned that it might take the fight to the World Trade Organisation, if the US continues to side with Boeing.
Earlier this month Canada scrapped plans to buy 18 Boeing Super Hornet fighter jets, underlining Canada's anger over the trade challenge.
The Commerce Department said it will collect the duties from the importer, if the US commission finds against Bombardier.
Delta has said it plans to move forward with the order, but does not expect to pay the tariffs.