UK jobs face being "crushed" by "nakedly political" trade sanctions against Bombardier's new airliner, large parts of which are built in Northern Ireland, according to unions.
The claim came from union Unite after the America's Commerce Department confirmed 292pc tariffs would be imposed on Bombardier's C-Series imported for sale to US airlines.
The duties came after Boeing said Canadian company Bombardier was selling 75 of its C-Series airliners to US carrier Delta at "absurdly low" prices, and was only able to do so because of illegal state support in Canada and the UK, according to the Daily Telegraph.
Boeing called for action from US authorities saying the C-Series competes with its small 737 jet, a point refuted by Bombardier.
Steve Turner, Unite assistant general-secretary, said: "The US Commerce Department decision on C-Series tariffs is nakedly political and has the potential to crush jobs, not only in Northern Ireland but in the US too.
"Boeing does not produce an aircraft in the same class as the C-Series and didn't even bid for the Delta contract it is complaining about, which leaves its complaint totally without merit."
Labelling Boeing's complaint "meritless" and a "cover to close the US market", Mr Turner called on the US International Trade Commission (ITC) to set aside the Commerce Department's decision when it hears the next stage of the complaint in February.
About a quarter of Bombardier's 4,000 staff in Belfast - where the company is the largest private employer - work on the C-Series and there are fears if the tariffs are imposed on the jet, it could kill the programme.
Unite said that as well as potentially costing UK jobs, the levies would harm the US economy, claiming half of the components for the jet are sourced in the US and the supply chain supports 22,000 American jobs. The high import tariffs would ramp up the cost of buying the C-Series, making the aircraft uneconomic for US buyers.
An ITC hearing earlier this week heard evidence from Bombardier executives claiming that since the tariffs were first mooted, potential sales to other US airliners have "frozen".
Unite was reacting to the latest development in the row between Bombardier and Boeing being played out in the US courts, which has blown up into a huge political battle.
Canada has abandoned an order for F-18 fighters from Boeing with Prime Minister Justin Trudeau saying his country would "not do business with companies who our economy harm". Canada has since warned that a future order from 88 fighters would assess the wider economic impact of the decision - a clear reference to Boeing.
Prime Minister Theresa May has asked Donald Trump to intervene and British MPs have called for reviews of multi-billion pound deals to acquire equipment from Boeing for the UK's armed forces.
Boeing has insisted the C-Series is a competitor to its 737 jet, adding that "investigations have established beyond question that Bombardier has taken billions of dollars in illegal government subsidies to prop up its C-Series programme… [with] Bombardier's unlawful actions harming US industry".