A Federal Court of Appeal judge has granted British Columbia intervener status in a legal challenge of the Trans Mountain pipeline expansion, while scolding the provincial government for its "blasé" approach to the case since taking power earlier this summer.

The New Democrats campaigned on using "every tool" available to kill the project and announced plans to join the Federal Court of Appeal case several weeks ago.

A series of plaintiffs including First Nations, environmentalists and local governments filed lawsuits challenging the federal government's approval of the $7.4-billion Kinder Morgan pipeline expansion between Alberta's oil sands and the Vancouver region.

Story continues below advertisement

Read more: What Kinder Morgan's Trans Mountain pipeline will mean for B.C.'s coast

Those lawsuits have all been merged into a single case, scheduled to be heard in October.

Justice David Stratas acknowledged that the spring election, which was followed by weeks of political uncertainty because of the minority government it produced, meant the province missed the formal deadline to apply for intervener status.

But Justice Stratas said the new government did not treat the case with enough urgency once in power.

"A number of aspects of British Columbia's motion are unsatisfactory," he wrote in a decision published on Tuesday. "For one thing, it took five weeks for British Columbia to bring this motion, a very long time in a closely-managed, expedited proceeding such as this."

Justice Stratas said the province's application to intervene did not articulate precisely what arguments it planned to advance. He also wrote that the provincial government "does not appear to understand the basic ground rules of the complex proceeding it is seeking to enter."

Nonetheless, he concluded the B.C. government should be heard. He said the province cannot introduce new issues or evidence that are not already part of the existing lawsuit, and he warned the government not to introduce politics into the case. Straying from those limitations, he warned, could prompt him to revoke the province's intervener status.

Story continues below advertisement

The B.C. government hired Thomas Berger, a former BC NDP leader acclaimed for his work as royal commissioner of the Mackenzie Valley Pipeline Inquiry in the 1970s, to provide legal advice in the case. Mr. Berger's name is listed on the court decision as the provincial government's lawyer.

The provincial Attorney-General's office could not immediately be reached for comment about the ruling. However, in a news release, Environment Minister George Heyman said it is "absolutely appropriate and necessary that we have the opportunity to defend British Columbia's interests in this very important case."

Kinder Morgan-owned Trans Mountain and the Alberta government both opposed British Columbia's application to intervene. They argued the province missed the deadline to make such an application. Trans Mountain also highlighted several "unsatisfactory features" of B.C.'s application, according to Tuesday's court decision, and said allowing the province to intervene now would introduce "new, complex issues" immediately before the case is about to begin.

The provincial government now faces the same Sept. 1 filing deadline as the other interveners, who have all had months to work on the case. Mr. Heyman's statement said the province would meet that deadline. The province also must pay Trans Mountain $7,500 to cover the costs associated with responding to the government's arguments.

The previous BC Liberal government conditionally approved the Trans Mountain proposal earlier this year after the federal government approved it.

Alberta Premier Rachel Notley and Prime Minister Justin Trudeau have warned that pipelines are a federal responsibility and that British Columbia has no authority to interfere with the Trans Mountain project.

Story continues below advertisement

B.C.'s NDP government is also seeking advice about a separate lawsuit in the B.C. Supreme Court in which the Squamish Nation is challenging the provincial Liberal government approval.

With a file from Canadian Press