The US government agreed with Boeings claims who said that the 110-to-130 seat Bombardier aircraft were dumped below cost in the US market.
The stage is now set for a loud and potentially destructive battle between Canadian and US interests, not just the companies involved but the governments as well - and this at a time when Ottawa and Washington (and Mexico) are locked in negotiations on NAFTA, the North American Free Trade Agreement.
And there's little doubt that Boeing was encouraged by Trump's aggressive language to seek the most punitive measures possible against Bombardier. The duty is expected to be in place before deliveries to Delta begin in April.
Boeing wasted no time in declaring victory on Tuesday. "It's imperative the British Government works to protect these vital jobs - the imposition of the tariffs could make Bombardier question remaining in Northern Ireland, where it has operations in four locations", Ms. McGregor said.
"The U.S. values its relationships with Canada, but even our closest allies must play by the rules", Commerce Secretary Wilber Ross said in a statement.
Boeing had been asking for an 80 per cent duty.
It called the magnitude of the USA duty "absurd and divorced from the reality about the financing of multibillion-dollar aircraft programs".
It's also extremely rich for Boeing to moan about government aid to a small competitor when it has benefited from decades of lavish support from the USA defence department.
May promised that the British government would continue working with Bombardier to protect the important jobs in Northern Ireland. He also once again threatened to cut government ties with Boeing.
For her part, the Prime Minister is said to have directly lobbied US President Donald Trump.
Prime Minister Justin Trudeau said Foreign Affairs Minister Chrystia Freeland raised the dispute with U.S. Trade Representative Robert Lighthizer during North American Free Trade Agreement talks in Ottawa.
Tuesday's finding was actually the first of two that the Commerce Department is scheduled to release on Bombardier.
In the meantime, Bombardier's sole USA contract for the CS100 is with Delta Airlines, and it wasn't expected to deliver the first of that 75-plane order until next spring.
That finding is scheduled on October 4, but could be delayed.
The decision can be appealed to the USA global trade court or under the dispute mechanism allowed in the North American Free Trade Agreement (NAFTA) that is now under renegotiation by the signatories - Canada, the United States and Mexico. If either body rules the opposite of the other, then no tariffs, or "countervailing duties" will be imposed.
Critics warned the ruling raised doubts about Britain's hopes of securing a quick trade deal with the United States when it leaves the European Union.
The Canadian government could also take the case to the World Trade Organization in Geneva.
POOL New/Reuters Quebec's Minister of Finance Carlos Leitao speaks as he tables his provincial budget at the National Assembly in Quebec City in 2014.
Quebec Premier Philippe Couillard called the Boeing-Bombardier row a low point in Canada-US relations.