A plane flies over a Bombardier plant in Montreal, January 21, 2014. REUTERS/Christinne Muschi

(Reuters) - Bombardier Inc BBDb.TO may have to wait to find out if its CSeries commercial jets will be hit by punishing U.S. duties after Boeing BA.N requested the U.S. Department of Commerce to postpone its preliminary ruling on its petition until Sept. 25.

In April, Boeing had asked the Commerce Department to investigate alleged subsidies and unfair pricing for Bombardier's CSeries airplane, accusing Bombardier of having sold 75 of the planes to Delta Air Lines Inc DAL.N last year at a price well below cost.

The allegations were denied by Bombardier.

The Commerce Department decided to launch an investigation into Boeing’s claims in May, a decision which was opposed by the Canadian government.

The U.S. Department of Commerce and Bombardier were not immediately available for comment, while Boeing declined to comment.