Theresa May is "bitterly disappointed" the US has opted to impose a tax on the C-Series jet made by Bombardier, one of Northern Ireland's biggest employers.
The PM said the UK would work with Bombardier to protect "vital" jobs after the US Department of Commerce proposed the 220% import tariff.
Rival Boeing complained Bombardier got unfair state subsidies from the UK and Canada, helping it win a major order.
Bombardier said it would fight the "absurd" ruling.
The Government and trade unions fear the imposition of tariffs could make Bombardier question remaining in Northern Ireland.
The ruling damaged the global aerospace industry and was "frankly not what we would expect of a long-term partner to the UK", said a UK government spokesman, while emphasising this was just the first step in a lengthy process.
About 1,000 jobs are linked to the C-Series, the wings of which are made at a purpose-built £520m factory at Queen's Island in Belfast.
The programme is not just important to Bombardier jobs in Belfast, but also to 15 smaller aerospace firms in NI - and dozens more across the UK - which make components for the wings.
The US Department of Commerce ruling, which could triple the cost of a C-Series aircraft sold into the United States, 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.
Bombardier - which employs some 4,100 in Northern Ireland, out of its total workforce of 28,000 - said the decision was "divorced from the reality about the financing of multibillion-dollar aircraft programmes".
The Canadian firm said Boeing was seeking to use US trade laws "to stifle competition".