British Columbia's new NDP government warned Kinder Morgan on Thursday that the company won't be able to begin construction on its Trans Mountain pipeline expansion, as the province pledged to join a legal challenge to ultimately kill the project.

The government's plan to oppose the project, a key election promise from the spring campaign, also lays the groundwork for a confrontation with the federal and Alberta governments, which both maintain the B.C. government has no power to block the $7.4-billion expansion from the Alberta oil sands to the Vancouver region.

"Our government made it clear that a seven-fold increase in heavy oil tankers in the Vancouver harbour is not in B.C.'s best interests," said Environment Minister George Heyman, a former executive director of Sierra Club BC.

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"Not for our economy, our environment, or thousands of existing jobs. We will use all available tools to protect our coastal waters and our province's future."

In the near term, Mr. Heyman said construction cannot begin on public land. He said the company has completed only three acceptable provincial environmental-management plans for the project, with five others not accepted because of inadequate consultation with First Nations.

"Until those plans are completed, Kinder Morgan, with the exception of private land, cannot put shovels in the ground," Mr. Heyman said.

The company has previously said some construction on the project will begin next month, but it won't actually involve work on the pipeline itself. Rather, it will include terminal construction and site preparation.

The province is also seeking intervener status in a case scheduled to be heard in the Federal Court of Appeal, expected in October. The court will hear several lawsuits launched by First Nations, environmentalists and others, which have been combined into a single case.

The government has hired Thomas Berger, a former BC NDP leader acclaimed for his work as royal commissioner of the Mackenzie Valley Pipeline Inquiry, to provide legal advice.

The government is also seeking advice about a separate lawsuit in the B.C. Supreme Court in which the Squamish Nation is challenging provincial approval by the previous BC Liberal government. The BC Liberals conditionally approved the project earlier this year after a separate approval from the federal government.

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Kinder Morgan said on Thursday that it continues to hope it can work out its differences with the B.C. government.

"We are committed to working with the Province and permitting authorities in our ongoing process of seeking and obtaining necessary permits and permissions," the company said in a statement.

"The Company takes seriously the comments from the Province of British Columbia and will be carefully reviewing their statements and the steps outlined today."

Grand Chief Stewart Phillip of the Union of BC Indian Chiefs said Thursday's announcement was significant – particularly the decision to not allow project work on public lands to proceed.

He said he believes the legal challenges and on-the-ground protests will ultimately be too much for the project to overcome.

"I'm absolutely convinced this pipeline will never see the light of day," he said in an interview.

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Alberta Premier Rachel Notley said B.C. had the right to join the legal case, but she repeated her insistence that pipeline approval is a federal issue.

"They're talking about getting independent legal advice about how to intervene in an already-existing court application. Fair enough. That's reasonable. That's what the courts are there for," Ms. Notley said at a ceremony to mark the start of construction of a different pipeline. The Alberta government also has intervener status in the federal case.

However, the decision on whether the line will go ahead has already been made by Ottawa, she said.

"The decision has been that the pipeline will go ahead because it is in the best interest of the country as a whole," she said. "There is a scope within which the provincial government has the ability to exercise its authority and that's what the provincial government is doing. But it is not attempting, in my view, to counteract the authority of the federal government to move forward on the pipeline that's been approved, that will be built."

A spokesman for Natural Resources Minister Jim Carr said the federal government made every effort to ensure the environmental-assessment process was fair.

"The decision we took on the Trans Mountain pipeline was based on facts and on evidence and what is in the national interest," Alex Deslongchamps said in an e-mailed statement.

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"We look forward to working with every province and territory to ensure a strong future for Canadians but the facts and evidence do not change."

The New Democrats campaigned on a promise to use "every tool in the toolbox" to kill the pipeline expansion, and the issue formed a key part of a power-sharing agreement with the BC Greens in the minority legislature.

The Greens immediately welcomed the announcement.

"In the B.C. Green caucus' view the National Energy Board process that led to this project's approval was profoundly flawed. Numerous questions remain unanswered or were simply dismissed," Green Leader Andrew Weaver said.

However, the BC Liberal opposition said the NDP steps suggest the government is opposed to the jobs expanding the pipeline would create.

The previous BC Liberal government under then-premier Christy Clark approved the pipeline earlier this year, saying the project had met a list of conditions she had laid out. Those conditions included environmental projections, consultations with First Nations and a "fair share" of economic benefits for the province.

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With reports from Jeff Jones, Sunny Dhillon and Shawn McCarthy