Alberta's top court said Tuesday that the federal government's environmental impact law is unconstitutional and Ottawa almost immediately announced its plan to appeal.
The Alberta Court of Appeal said the Impact Assessment Act is an "existential threat" to the division of powers guaranteed by the Constitution.
Prime Minister Justin Trudeau, later in the House of Commons, said that the law delivered on a promise to reform a "broken system and restore public trust in how decisions about major projects are made."
"We will be appealing this decision."
The Alberta government, calling it a Trojan Horse, had challenged the act over what the province argued was an overreach into provincial powers.
The act, given royal assent in 2019, lists activities that trigger an impact review and allows Ottawa to consider the effects of new resource projects on a range of environmental and social issues, including climate change.
Alberta argued the law could use those concerns to greatly expand the range of federal oversight into areas of provincial jurisdiction.
A majority of five justices, giving their legal opinion in a 204-page document released Tuesday, sided with Alberta.
"Climate change constitutes an existential threat to Canada," Chief Justice Catherine Fraser and two others wrote.
"The (Impact Assessment Act) involves another existential threat — one also pressing and consequential — and that is the clear and present danger this legislative scheme presents to the division of powers guaranteed by our Constitution and, thus, to Canada itself."
The opinion added that legitimate concerns about the environment and climate change should not override the division of power.
"If the federal government believes otherwise, it should make the case for an increase in its jurisdiction to the Canadian public."
A fourth judge signed off on that opinion with the exception of one section.
In a dissenting opinion, Justice Sheila Greckol said the federal environmental impact law is a valid exercise of constitutional authority.
"The federal environmental assessment regime … prohibits projects … that may have effects in federal jurisdiction — on fish and fish habitat, aquatic species, migratory birds, on federal lands or federally funded projects, between provinces, outside Canada and with respect to Indigenous peoples," she wrote.
"The complexities and the urgency of the climate crisis call for co-operative interlocking environmental protection regimes among multiple jurisdictions, each functioning at its highest and best within their constitutional jurisdiction."
Now is not the time to "give credence to any kind of 'Trojan Horse' metaphor advanced by Alberta and Saskatchewan," Greckol wrote. "Likening Canada to a foreign invading army deceptively breaching our protective walls, only fuels suspicion and pits one level of government against each other."
There were 17 interveners in the case.
Alberta was supported in its challenge by the governments of Saskatchewan and Ontario, as well as three First Nations and the Indian Resource Council.
Seven of the interveners, including a wide array of environmental and legal groups as well as other First Nations, were in support of Ottawa.