The UK Government has been accused of impotence after the US upped a proposed import levy on products made by one of Northern Ireland's largest employers to 300%.
Unite union assistant general secretary Steve Turner accused the UK of "a betrayal of its responsibility" to workers during the dispute between American aerospace giant Boeing and Bombardier.
Canadian-owned multinational Bombardier, which employs more than 4,000 people in Belfast, has been hit by a proposed 80% levy on exports to its southern neighbour, the US Department of Commerce has confirmed.
This comes on top of a planned 220% tariff on its aircraft previously announced in a row over state aid.
Mr Turner said: "This latest threat of additional tariffs by the US government, raising to 300% the tariffs imposed so far, highlights an impotence on behalf of a UK Government whose ministers through their inaction appear happy to put Trump's 'America first' policy ahead of UK manufacturing jobs.
"Ministers sat in Westminster cannot shrug off the Bombardier crisis as a trade spat between Canada and the US. This directly affects tens of thousands of jobs across the UK."
Bombardier is due to begin delivering a blockbuster order for up to 125 new jets to Atlanta-based Delta Airlines next year.
The wings for the new aircraft are made at Bombardier's plant in the DUP stronghold of East Belfast.
UK Prime Minister Theresa May had lobbied US President Donald Trump over the dispute 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 prices in the US.
The subsidy row arose after Northern Ireland's powersharing administration and the UK Government pledged to invest almost £135 million (€150 million) in the establishment of the C-Series manufacturing site in Belfast.
The programme also received a 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.
Mr Turner accused Boeing of "a clear attempt to use its corporate power in a worryingly protectionist United States to put a legitimate competitor out of business".
He added: "Ministers have a duty to defend jobs and future manufacturing in Northern Ireland and the rest of the UK by demanding Boeing attends an urgent summit involving Prime Minister Theresa May and the Canadian prime minister Justin Trudeau alongside the unions representing our workforce and withdraws its unwarranted claims.
"A failure to do so must lead to similar tariffs and sanctions on Boeing's current and future work on behalf of the UK Government and across the EU to protect our wider aviation sector from further protectionist assault in what could quickly become a trade war with an increasingly protectionist US regime."
Liberal Democrat leader and former business secretary Sir Vince Cable said the row was a "blow" to Mrs May's hopes of a good trade deal with the US after Brexit.
"It is a fantasy that the UK can promote free trade in partnership with a US government which believes in economic nationalism," Sir Vince said.
"The Government must now get out there and fight for the UK's manufacturing jobs."