The UK government told US authorities that it did not consider itself a “legally proper party” to the Bombardier trade dispute with Boeing.
It also only submitted four pages of legal argument to a key hearing on the case held in December.
The UK government says it has remained “proactive from the outset”.
The US Commerce Department proposed tariffs of 292% on any imported planes.
On Friday, the International Trade Commission (ITC) will rule if Boeing has been harmed by the subsidies.
If the commission decides the US-based firm has been harmed, then the tariffs will come into effect and the C-Series will effectively be shut out of the all-important US market – with implications for thousands of jobs in Belfast.
A BBC NI Spotlight investigation uncovered a substantial difference in approach to the case between the UK and Canadian Governments.
While the UK submission to the ITC was only four pages long, the Canadian submission ran to more than 170 pages.
“It’s clear that the UK government has not come in full force, certainly not at the International Trade Commission,” former ITC Commissioner, Prof Jennifer Hillman, told Spotlight.
The programme reveals that the government told authorities at the US Department of Commerce that it didn’t consider itself a “legally proper party” to the dispute.
Although UK government lawyers answered many lengthy and detailed questions put to them by the US Department of Commerce officials, a number of replies were late – with varying excuses offered when requesting extra time.
In one letter, a UK official offers the excuse that the Department for International Trade is a new department in a new building, and “many of the files which need to be examined are still packed”.
In another letter to the US Department of Commerce, lawyers for the UK offer the “Twelfth Fortnight” in Northern Ireland as a reason for not being able to reply on time.
“It is common practice for employees to take longer vacations during the weeks surrounding the 12th (known as the “Twelfth Fortnight”),” the letter states.
“And the employees in the government offices in Northern Ireland responsible for gathering documents and providing information in response to the Department’s questionnaire will be out of the office for the remainder of this week.”
Business Secretary Greg Clark rejects the accusation that the government’s approach has been less than “full force”.
“Quite the reverse, right from the outset, we have worked vigorously,” he told the programme.
“Personally, I have never seen such a high level, consistent level of engagement.”
But Shadow Secretary of State for Northern Ireland Owen Smith is critical of the government’s approach and submission to the ITC.
“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.
Spotlight’s full investigation airs on Wednesday at 20:30 GMT on BBC One NI and at 23:25 on the BBC News Channel. It will also be available on the BBC iPlayer.
A spokesman for the department said: “The UK government has remained proactive from the outset of the Bombardier dispute, working with the Canadians to ensure all evidence provided is thorough and robust.
“This includes the UK submitting more than 7,000 pages of evidence to the US International Trade Administration and International Trade Commission.”