A British Columbia First Nation is questioning the greenlighting of the Kinder Morgan Trans Mountain pipeline, arguing there may be new evidence the federal government failed to complete consultations with First Nations prior to approving the expansion.

In a letter filed to the Federal Court of Appeal on Thursday, the group's lawyer, Scott Smith, says two stories published in April by the National Observer, a national news website, confirmed the Tsleil-Waututh Nation's fears that the government had acted improperly.

Citing documents laid out in the two media reports, Smith's letter says that government employees were instructed to give cabinet a legally sound basis to approve the pipeline at the same time the government was still supposedly consulting with First Nations and had not reached a decision yet.

The Tsleil-Waututh Nation intends to bring forward an urgent motion requesting that the federal government be ordered to produce the documents and if necessary, will bring another motion to allow new evidence to be heard in court in order to ascertain whether the government breached its duty to consult with the Tsleil-Waututh Nation.

"Canada represented to the parties and the Court that it had appropriately and meaningfully consulted with TWN … when it now appears Canada had already pre-determined that it would approve the Project," the letter says.

Finance Minister Bill Morneau is tasked with saving the Trans Mountain pipeline. He speaks with CBC’s Wendy Mesley about the ongoing battle to get the pipeline expansion built, and how the outcome will affect the Liberals come election time. 6:33

This new potential evidence of "bad faith" and "dishonourable conduct" could have bearing on the court's decision as to whether the government discharged its duty to consult, Smith's letter states.

The first motion is expected to be filed next week, a source with knowledge of the court proceedings told CBC News.

Should that motion be approved, it would likely add more than a month to the timeline for the case.

Last fall, seven First Nations, the cities of Burnaby and Vancouver, and two environmental groups asked the court to overturn the federal government's decision to approve the expansion of the $7.4-billion pipeline from Edmonton to Burnaby.

This new letter is unusual, since the case — which included the Tsleil-Waututh Nation — was heard in the fall. However, Smith says in the letter there is legal precedent for the court to reopen the matter.

Pushback from government, pipeline company

In a written response from the federal government obtained by CBC News, it outlines it has no position on whether the First Nation should be "permitted to bring this motion at this late date, over six months after the hearing of these proceedings has concluded."

That letter also explains the government does not see the motion as "urgent," but asks the court that if the motion is allowed to proceed that the timeline be brief — 10 days for a government response after the motion, and four days for the Tsleil-Waututh Nation to reply.

In a statement provided to CBC News, Kinder Morgan said this motion would cause unnecessary delay to the case, as a decision was expected this spring.

The company argues reopening the case now would cause "significant prejudice" against Trans Mountain. In their own letter to the court, they ask for the proposal from the Tsleil-Waututh Nation to be rejected.

While 43 First Nations have financial benefit agreements with Kinder Morgan in support of the new pipeline, the Tsleil-Waututh and several other nations along its path oppose it.

The Trudeau government approved the project in November 2016 after the NEB recommended it get the green light, subject to 157 conditions.

So far, the NEB has approved 66 per cent of the project's proposed detailed route and hearings are scheduled to continue on the project into the fall, according to the regulatory body.

Kinder Morgan has engaged with 133 Indigenous communities and groups in Alberta and B.C. over the last six years on the project.