British Columbia cannot lay solitary claim to western tidewaters and must allow landlocked Alberta to have access to the coast for export markets, says Alberta Premier Rachel Notley.

"At the end of the day, we can't be a country that says one of its two functional coastlines is only going to do what the people who live right beside it want to do," Notley said in an interview Wednesday with CBC Radio's Edmonton AM.

"We have to be able to engage in international trade, and that's what we're doing."

British Columbia's New Democrats and Greens signed a four-year political manifesto Tuesday with a long list of ambitions to govern the province, including a plan to stop the Kinder Morgan pipeline expansion.

The $7.4-billion project would triple the capacity of the line, which runs from Edmonton to Burnaby, B.C., providing Alberta critical access to international export routes.

NDP Leader John Horgan, who would become B.C. premier under the agreement with the Greens, said both parties have a responsibility to "defend" the coastline and stop the pipeline.

The line, which opponents say would increase tanker traffic seven-fold off the West Coast, has faced opposition from environmental and Indigenous groups who fear increased crude oil exports would threaten B.C's fragile coastal waters.

The province's new political alliance has only added to those tensions.

Even before the alliance was announced, both federal and provincial leaders sparred over the pipeline proposal with Andrew Weaver, leader of the B.C. Green Party, who downplayed any future economic benefits to B.C.. He told reporters the prospect that thousands of promised pipeline jobs would materialize was as likely as "unicorns in all our backyards."

'There is not really a Plan B'

Despite the aggressive political posturing among her counterparts to the west, Notley remained adamant the pipeline will proceed.

The pipeline has been approved by the National Energy Board and no province, or newly formed political alliance, has the power to veto those decisions.

It's critical that Alberta oil be granted access to the B.C. coast, Notley reiterated on Wednesday.

Canada is made up of 10 provinces and three territories with a shared commitment to each other, Notley said.

"And that's why our Constitution sets out that the federal government has the ultimate responsibility for infrastructure projects," said Notley, who noted that her government has seen no need to draft any contingency plans for oilsands exports. 

"There is not really a Plan B. Once the decision is taken, the work will go forward."

'Letter of the law behind her'

Though Notley's arguments are well-grounded in law, growing political tensions could still stymie progress on the expansion, warned Eric Adams, an associate professor of law at the University of Alberta.

Under the Constitution, Prime Minister Justin Trudeau has the power to declare the project a "general advantage of Canada," which would allow Ottawa to assume control.

But enacting those constitutional powers could trigger political outrage among provincial and municipal governments, which have wide-ranging jurisdictional powers, said Adams

"She's got the letter of the law behind her," Adams said of Notley's arguments about the pipeline's regulatory approval.

"But that doesn't mean that B.C. doesn't have its own jurisdiction to set its own conditions over environmental standards, over construction permits."

The B.C. government could still create many regulatory impediments to the pipeline, through the courts and the environmental permit process, said Adams.

"I think we're going to see a lot of tension and constitutional politics and maybe some litigation in the months and years [ahead] about where B.C. can set other kinds of conditions that makes life difficult for that pipeline. It's a complicated picture."

With files from Reuters