By Doug Cameron
Published: Sept 26, 2017 9:21 p.m. ET

U.S. officials said they plan to slap a large tariff on sales of a new Bombardier Inc. jetliner to U.S. airlines, inflaming a simmering trade spat with Canada.

The International Trade Commission ruled in favor of a complaint from Boeing and said it would add a 220% tariff to the cost of the new CSeries jet. The jet’s sale to Delta Air Lines Inc. DAL, +1.24% last year prompted a complaint from Boeing.

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.

“The Commerce Department will instruct U.S. Customs and Border Protection to collect cash deposits from importers of 100- to 150-seat large civil aircraft based on these preliminary rates,” the department said in a statement.