(Adds share prices, comments from former ITC chairman, Bombardier)
WASHINGTON/MONTREAL, Jan 26 (Reuters) - Canada's Bombardier Inc on Friday won an unexpected trade victory against U.S. planemaker Boeing Co as a U.S. agency rejected imposing hefty duties on sales of Bombardier's new CSeries jet to American carriers.
Shares of Bombardier, which relies financially on sales of the jet, rose 15 percent. Boeing shares fell slightly.
The U.S. International Trade Commission voted 4-0 to reject Boeing's claims that it suffered injury by Bombardier underpricing the CSeries in the U.S. market and discarded a Commerce Department recommendation to slap a near 300-percent duty on sales of the 110-to-130-seat jets for five years.
The ITC had widely been expected to side with Chicago-based Boeing, the world's largest maker of jetliners, which accused Bombardier of dumping the planes, or selling them below cost, in the U.S. market.
The ITC, which currently has four commissioners, did not give an explanation on Friday for its decision.
In a statement, Bombardier called the decision a "victory for innovation, competition, and the rule of law," and a win for U.S. airlines and the traveling public.
Boeing said it was disappointed that commission did not recognize "the harm that Boeing has suffered from the billions of dollars in illegal government subsidies that the Department of Commerce found Bombardier received and used to dump aircraft in the U.S. small single-aisle airplane market."
Through a venture with European planemaker Airbus SE , which has agreed to take a majority stake in the CSeries this year, Bombardier plans to assemble CSeries jets in Alabama to be sold to U.S. carriers starting in 2019.
The case has sparked trade tensions between the United States and its allies Canada and the UK. Ottawa last year scrapped plans to buy 18 Super Hornet fighter jets from Boeing.
Former ITC chairman Dan Pearson praised the decision by phone.
"Not a single commissioner was willing to buy Boeing's arguments," he said. "I think 'America First' is a policy of the White House and the Commerce Department," he said. "But it's not the policy of an independent agency (like the ITC)." (Reporting by David Shepardson, Lesley Wroughton and Allison Lampert; editing by G Crosse and Bill Rigby)