The US Department of Commerce has ruled against the aerospace firm Bombardier in its dispute with rival Boeing.
An interim tariff of 219.63% has been proposed on the import of Bombardier’s C-Series jet to the US.
Boeing had complained Bombardier got unfair state subsidies from the UK and Canada, helping it win a major order.
The government and trade unions fear it might even make Bombardier question remaining in Northern Ireland, where it employs 4,100 people in four locations.
The case will now be considered by the US International Trade Commission for final ruling in February.
It could potentially jeopardise a major order made last year from US airline Delta – a $5.6bn (£4.15bn) deal for up to 125 of the jets.
About 1,000 jobs at the company’s Belfast operation are linked to the C-Series.
Bombardier – which bought Short Brothers in 1989 – is one of Northern Ireland’s largest employers.
The wings of the C-Series are made at a purpose-built £520m factory at Queen’s Island.
The programme is not just important to Bombardier jobs in Belfast, but also 15 smaller aerospace firms in NI – and dozens more across the UK – which make components for the wings.
The Northern Ireland Executive and UK Government pledged to invest almost £135m in the establishment of the C-Series manufacturing site.
The programme received £750m from Quebec’s provincial government in 2015 when its fortunes appeared to be ailing.