It's affirming Boeing Co's complaint that the Canadian company received illegal subsidies and dumped the planes at "absurdly low" prices.
The decision underscored the defensive trade policy of U.S. President Donald Trump, and could effectively halt sales of Bombardier's innovative new plane to U.S. airlines by quadrupling the cost of the jets imported to the United States.
The Commerce Department proposed a 79.82 percent antidumping duty on Friday, on top of a 219.63 percent duty for subsidies announced last week.
The new duty follows a preliminary finding that Bombardier sold 75 CSeries jets below cost to Delta Air Lines Inc in 2016. The total was well above the 80 percent Boeing sought in its complaint.
The proposed duties would not take effect unless affirmed by the U.S. International Trade Commission (ITC) early next year.
The duties are expected to heighten trade tensions between the United States, Canada and Britain, where CSeries wings are made. The United States, Canada and Mexico also are negotiating to modernize the North American Free Trade Agreement.
After the first duty was announced on Sept. 26, Canada and Britain threatened to avoid buying Boeing military equipment, saying duties on the CSeries would reduce U.S. sales and put thousands of Bombardier jobs in their countries at risk.
Bombardier shares were last up 0.5 percent to C$2.20. Some analysts said the muted response reflected a view that the penalties might not actually be applied.
Boeing, the world's largest plane maker, hailed the decision and hinted at an alternative for Bombardier.
"These duties are the consequence of a conscious decision by Bombardier to violate trade law and dump their CSeries aircraft to secure a sale," Chicago-based Boeing said in a statement.
"Bombardier always has the option of coming into full compliance with trade laws," Boeing added.
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