UK Prime Minister Theresa May is "bitterly disappointed" by a US Department of Commerce decision to impose a tariff of nearly 220% on a new model of passenger jet manufactured by Bombardier, one of the North's biggest employers, Downing Street said.
The comment came after Mrs May was accused of being "asleep at the wheel" as unions warned that thousands of jobs could be put at risk by the US move.
Bitterly disappointed by initial Bombardier ruling. 1/2— UK Prime Minister (@10DowningStreet) September 27, 2017
The Government will continue to work with the company to protect vital jobs for Northern Ireland. 2/2— UK Prime Minister (@10DowningStreet) September 27, 2017
More than 4,000 people are employed in Belfast by Canadian multinational Bombardier and thousands more jobs in the North are supported through the manufacturer's supply chain, according to trade unionists.
Mrs May had lobbied US President Donald Trump over the dispute, which was sparked by complaints from rival Boeing that Bombardier received unfair state subsidies from the UK and Canada, allowing the sale of airliners at below cost price in America.
The Democratic Unionist Party (DUP), which the Tories rely on to swing key Commons votes for the Government, has repeatedly pressed the Prime Minister over the issue.
The controversial US decision came as Foreign Secretary Boris Johnson was using the launch of a new think-tank on Wednesday to push the cause for global free trade.
Announcing the regulator's preliminary finding on Tuesday, US Secretary of Commerce Wilbur Ross said the subsidisation of goods by foreign governments was something that President Trump's administration "takes very seriously".
Bombardier labelled the determination "absurd", while in its response the UK Government said the statement was "disappointing" and pledged to defend British interests "at the very highest levels".
However, unions warned that the preliminary determination was "unlikely" to be overturned by Mr Trump, who has been clear in his aim to fiercely protect American jobs, casting a shadow over the industry's future in the North.
Ross Murdoch, the GMB union's national officer, said the initial ruling was a "hammer blow" to Belfast and risked sending shockwaves through the North's economy.
"Theresa May has been asleep at the wheel when she could and should have been fighting to protect these workers. It's high time she woke up," he said.
Another 9,400 supply chain jobs could be wiped out in the North on top of those directly employed at the plant, Mr Murdoch warned.
"That's 14,000 people in Northern Ireland now in jeopardy," he said.
Unite union regional secretary Jimmy Kelly said: "The decision taken by the US Department of Commerce was not unexpected - unfortunately it is unlikely to be overturned by President Trump, whose protectionist tendencies are well known.
"The threat of punitive tariffs on the C-Series will cast a shadow over Bombardier's future unless the company can source alternative and substantial sales outside the US market."
DUP leader Arlene Foster pressed Mrs May to raise the issue with Mr Trump when the two met in New York earlier in September.
Mrs Foster said the DoC's determination was "very disappointing", but added that it was not the end of the process.
"The C-Series is a hugely innovative aircraft that is vital to Bombardier's operations in Belfast," she said.
"It is this innovation that sets the C-Series apart and it is not in direct competition with Boeing."
The alleged unfair subsidy arose after the North's powersharing administration and the UK Government pledged to invest almost £135 million in the establishment of the C-Series manufacturing site in Belfast.
The programme also received one billion US dollars from the Canadian provincial government in Quebec in 2015 when its fortunes appeared to be ailing.
Boeing's complaint said it was seeking a "level playing field" for global competitors, but Bombardier accused its rival of hypocrisy.
The operation's immediate future was thought to have been secured after Bombardier signed a 5.6 billion US dollar deal in 2016 to provide the aircraft.
The manufacturer, which has been a major employer in the North for 30 years, is due to begin delivering a blockbuster order for up to 125 new C-Series jets to Atlanta-based Delta Airlines in 2018.
Trade unionists expect a final ruling on the pricing policy to be made in February and the regulator said it will continue to evaluate its decision.
The Department of Commerce's enforcement and compliance unit is responsible for vigorously enforcing US trade laws and does so through an impartial, transparent process that abides by international rules and is based solely on factual evidence, its statement said.
It added: "Imports from companies that receive unfair subsidies from their governments in the form of grants, loans, equity infusions, tax breaks and production inputs are subject to 'countervailing duties' aimed at directly countering those subsidies."
Shadow education secretary Angela Rayner called on the Government to protect the jobs involved.
She told BBC Radio 4's Today programme: "The Bombardier situation is quite a crucial one because these are UK jobs that we depend on, thousands, and actually the Government were involved in that contract process and were robust and I think that we have to defend the fact that that contract was awarded and we followed the rules."
DUP MP Sir Jeffrey Donaldson said the decision could have a "devastating" impact on the Belfast economy, but said he believed "David can defeat Goliath".
He told BBC Radio 4's Today programme: "Clearly it's critical for Belfast, 4,000 jobs and a further 80 companies in the supply chain, this would have devastating consequences on our economy if the factory were to close and we're going to do everything we can to ensure that doesn't happen."
He said the Prime Minister had been working with Canada to lobby on this issue, and he believed Bombardier had a strong case.
He added: "C-Series is an innovative aircraft, it's hugely competitive and I think that's why Boeing, in the most unreasonable and irrational way, have gone to try and block Bombardier from the market."
He went on: "I think the Prime Minister is putting down a marker that if Boeing want to do business in the UK, then they should stop acting in such an unreasonable and irrational manner against an aircraft that is innovative that is not in direct competition with Boeing, and if America is about free trade and free enterprise then it really shouldn't be doing this."
Haley Dunne, director of public affairs at Bombardier, described the move as "absolutely absurd" and said it showed this was not the right process that should be followed for trade disputes.
She told the BBC: "This is an absurd reaction to be honest, we obviously are very disappointed with the preliminary decision.
"As we said it's a preliminary decision and we'll continue through the process and we are not surprised that the petitioner is favoured at this stage, but the magnitude of the binds that are being imposed is absolutely absurd."
Asked if the future of the factory was at risk, she added: "There's no doubt that the C-Series is key to our future....absolutely the C-Series programme is critical to our future here in Belfast."
She added it was a bit of a "David and Goliath" dispute.
There are fears for thousands of jobs in the North after its biggest private employer was hit by huge US trade tariffs.
America wants to make it harder for Bombardier to export a model of airplane - after a complaint from rival Boeing that it has been getting State subsidies.
Boeing claims the company has been getting illegal subsidies from the UK and Canada and selling jets at less than what it costs to make them.
4,000 people are employed at its site in Belfast.
Northern Ireland Secretary James Brokenshire says Theresa May is taking the issue very seriously.
"The Prime Minister has conveyed a clear message to the US President now on two occassions as to the significance of the Bombardier case to Northern Ireland and to the workers who are employed here," he said.