The ruling could put as many as 4,000 jobs in Belfast, where parts of the C-Series are made, at risk and potentially threaten the fragile coalition government in the U.K. which Prime Minister Theresa May brokered earlier this year with members of Northern Ireland's Democratic Unionist Party (DUP).
The 219.63% duty on new commercial jets made by Bombardier and sold in the United States would effectively triple the $80 million base cost of the C-Series and likely lead to its scrapping. Montreal-based Bombardier called the ruling "absurd" and vowed to challenge it through the International Trade Commission.
"The U.S. values its relationships with Canada, but even our closest allies must play by the rules," said Commerce Secretary Wilbur Ross
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