The battle of aerospace rivals Bombardier and Boeing will enter a critical phase this week.
The two aircraft makers will appear before the U-S International Trade Commission today, to argue their respective cases.
Boeing claims it was hurt by Bombardier's multibillion-dollar deal to sell up 125 C-Series passenger jets to Delta airlines.
The U-S Department of Commerce proposed a 300 per cent duty after finding Bombardier received government subsidies that allowed it to sell its C-Series planes at an unfairly low price.
Montreal-based Bombardier, however, insists the Delta deal had no impact on Boeing's financial health.
Canada's ambassador to the U-S, David MacNaughton, along with his British counterpart, are expected to attend Monday's hearing on behalf of their governments, which both support Bombardier.