It is thought that if the ITC rules against Bombardier and no further action is taken the tariffs could be imposed on C-Series jets sold in America within weeks.
If this happens, then sales to launch customer Delta are likely to be cancelled and the key US market will effectively be closed to Bombardier, throwing into doubt jobs at the company's Belfast plant, which is the region's biggest private employer.
The row over the C-Series has drawn in national leaders from the UK, Canada and US.
Prime Minister Theresa May called US President Donald Trump asking him to personally intervene and Canadian Prime Minister Justin Trudeau has cancelled a multi-billion order for F-18 fighters from Boeing and warned the company off another bigger contract for combat jets.
Trudeau also said his country would not do business "with a company that is trying to sue us, eliminate tens of thousands of jobs and put our companies out of business".
Bombardier argues that subsidies to fund development of new airliners such as the C-Series are common around the world, along with heavy discounts for the first sales of new types such as the C-Series.
It also claims that Boeing does not produce an aircraft of the same size which could compete for the Delta deal so the US aerospace company could not argue that it had suffered by Delta picking its aircraft.
The case has been further complicated by Airbus swooping in and taking a 50.01 per cent stake in the C-Series with promises to build the airliner at its plant in Alabama, a move Bombardier says "will create hundreds of US jobs – and eliminating the possibility of Boeing being harmed by C-Series imports".
Boeing says the Delta deal harmed sales of the smallest versions of its best-selling 737 jet, which could have competed, and the subsidies are prohibited in any case.
The US giant the dispute is "not political, but legal" involving "a textbook case of dumping" with the state support enabling the C-Series to be sold at below the cost of production.
Britain's involvement is made even more complex by Boeing's growing footprint in the UK, as it employs 2,200 staff here directly and supports a further 16,000 British jobs. It is also opening a new factory in Sheffield.
Huge defence contracts for Boeing-built spyplanes and attack helicopters also create issues from the UK, which is depending on the company to equip its armed forces.
MPs have brought up these deals for P-8 Poseidon reconnaissance aircraft and Apache helicopters, questioning whether they can be stopped in retaliation.
Plans for an appeal - which could further string out the dispute which began last spring - emerged as an investigation by BBC Northern Ireland discovered that the UK Government did not see itself as a "legally proper party" to intervene in the dispute.
The report claimed the UK government submitted only four pages of argument to a crucial hearing about the case. Canada turned in a 170-page document to argue its case.
Owen Smith, Shadow Secretary of State for Northern Ireland, hit out at the the Government's submission to the ITC hearing. "They have been more concerned with the optics of looking to be doing a good job, defending jobs here in Belfast, rather than doing so," he said.
Business Secretary Greg Clark defended the UK Government's role, saying "from the outset it worked vigorously" and that he had never before seen such "a high level, consistent level of engagement".
