OTTAWA -- The bitter dispute between Bombardier and Boeing will enter a critical phase next week, when the two aerospace rivals appear before an all-important trade tribunal -- whose ruling will ultimately decide the fight.

The U.S. International Trade Commission will hold hearings Monday in which Boeing will explain why it believes it was hurt -- or could be hurt -- by Bombardier's landmark deal to sell C-Series passenger jets to a U.S. airline.

Bombardier will have its own chance to fire back by arguing that the multi-billion-dollar deal involving up to 125 planes had no impact on Boeing's economic well-being.

Canada's ambassador to the U.S., David MacNaughton, along with his British counterpart, are also expected to attend on behalf of their governments, which support Bombardier.

The hearings mark the last chance for all sides to try and sway the commission before it issues a final ruling, likely in February, which will determine whether every C-Series jet entering the U.S. is hit with a hefty duty.

The U.S. Department of Commerce proposed a 300-per-cent duty after finding that Bombardier broke trade rules, but the penalty will only apply if the trade commission sides with Boeing.