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”.
‘Things could get worse’
Julian O’Neill, Northern Ireland Business Correspondent
The outcome was predicted – but not the severity of the penalty.
The tariff could triple the cost of C-Series aircraft in the US, effectively killing the market for Bombardier.
It also puts a major order with Delta Airlines at risk – and things could get worse before they get better.
Washington’s Department of Commerce is due to make a second tariff ruling on 5 October.
But a more important date is next February when the US International Trade Commission will either uphold the penalty or remove it.
This was always the focus for Bombardier and its allies.
The firm said it had created a “superior aircraft” that is more efficient and comfortable but Boeing was trying to prevent “US passengers from realising these benefits, irrespective of the harm that it would cause to the US aerospace industry and the cost to airlines and consumers”.
But Boeing said the dispute was about “maintaining a level playing field”, and said its aim was to make sure that “aerospace companies abide by trade agreements”.
Delta, however, said there was no risk to US businesses as neither Boeing nor any other US firm produced 100-110 seat aircraft to compete with the C-Series.
US Secretary of Commerce Wilbur Ross said one reason for the tariff ruling was Bombardier’s failure to cooperate with the investigation.
“The evidence we have to rely on is evidence not coming from Bombardier but evidence being proposed by Boeing and other outside parties,” he said.
The US government was “not necessarily” targeting Bombardier’s Northern Irish factory, he said, but added: “If you’re building wings for a plane that doesn’t get built, that’s a problem.”
Analysts say that the ruling will increase tensions between the US government and Canada and the UK, both countries have deals to supply military aircraft worth billions of dollars with Boeing.
The UK Government and Northern Ireland Executive 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.
Jimmy Kelly, regional secretary of the Unite union, said the decision to impose a tariff posed a “direct and very serious threat” to Bombardier jobs in Belfast, as well as many more that are dependent on the company.
Angela McGowan, director of CBI Northern Ireland, said the ruling placed an important driver of Northern Ireland’s economic growth at risk.
Northern Ireland has effectively been without a devolved government for almost six months, and Ms McGowan said the situation reinforced the need for a return of a power-sharing executive.
“With jobs and future prosperity in the region being put at risk by decisions made far away from Belfast, we need a devolved government that can speak up for and champion the needs of the local workers and businesses most affected,” she said.