The department suggested the tariffs could be as high as 219 percent.
Boeing had recommended a heavy tariff on the C Series, paid by Delta or any US airline importing the aircraft in what would be considered a major blow to Bombardier and its ability to establish the new jet in the world's largest aviation market.
The Commerce Department ruled that Bombardier had unfairly benefited from state subsidies in selling its 100- to 150-seat aircraft below cost to Delta Airlines. -Canada relations. Nonetheless, the Quebec government will continue to promote open markets, he said.
Prime Minister May also expressed concerns saying she was "bitterly disappointed" with the development, adding the United Kingdom would work with Bombardier to protect "vital" jobs.
Union representatives in the United Kingdom were quick to call on the United Kingdom government to take counter measures to safeguard the jobs in Northern Ireland.
"This does not bode well for a potential trade agreement british-american", said Hugo Swire, a conservative mp former secretary of State for foreign Affairs.
Bombardier supports hundreds of other jobs through its suppliers and has been a major employer in Northern Ireland for nearly 30 years. That decision is expected to be made next year. But British and Canadian leaders have publicly urged the Trump Administration not to impose tariffs.
Even before the ruling was announced, Canadian Prime Minister Justin Trudeau said that if the dispute continued, he might cancel a multibillion-dollar deal to buy Boeing's F/A-18 Super Hornet fighter jets.
"We will always defend Canadian companies and Canadian workers against unfair and costly protectionism".
Some have questioned why Boeing would wade into worldwide geopolitics in the first place.
What are the aerospace companies saying about it?
"This is a risky moment for Boeing", Aboulafia said.
"The simple truth is that Bombardier created a superior aircraft that is more efficient, more comfortable, and quieter", said the company.
Boeing hailed the decision in a statement.
The U.S. agency's decision late Tuesday added to investor uncertainty over the fate of Montreal-based Bombardier after two of its main rivals in the global rail market agreed to combine their transport operations, triggering a double-digit gain in its share price during Tuesday trading.
"This would have devastating consequences on our economy if the factory was to close and we are going to do everything we can to ensure that doesn't happen. if America is about free trade, and free enterprise it really shouldn't be doing this". According to the GMB union it supports almost 10,000 further jobs through its suppliers. Over the summer, the Trump administration called for tariffs on imports of softwood lumber.
The row has come at a bad time for May, who was severely weakened by her party's poor showing in an election in June and who has been struggling to contain infighting within her top team over Brexit.
That could not only drag the case out, but also leave a cloud of uncertainty hovering over Bombardier, and affect its ability to sell more planes into the US market or overseas.
The Washington Post's Christian Davenport contributed.
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