The decision by the US Commerce Department announced on Tuesday, follows a complaint by American manufacturer Boeing, which had claimed that Bombardier unfairly benefited from state subsidies in selling its C Series aircraft below cost to Delta Airlines.

Commerce Secretary Wilbur Ross said Washington valued its relationships with Canada, but even the closest allies of the United States must play by the rules.

"The subsidization of goods by foreign governments is something that the Trump administration takes very seriously, and we will continue to evaluate and verify the accuracy of this preliminary determination," he added.

Both Canada and the United Kingdom, where some of the parts are built, had sought to persuade the US to drop the case. Now, the ruling — which is set for a final determination on December 12 — could further strain relations between Ottawa and Washington, already at a low point after US President Donald Trump pushed for a redraft of the North Atlantic Free Trade Agreement (NAFTA) with Canada and Mexico.