Alberta's top court said Tuesday the federal government's environmental impact law is unconstitutional, and Ottawa almost immediately announced its plan to appeal.

The Alberta Court of Appeal said in its non-binding opinion in the reference question concerning the Impact Assessment Act that the controversial law is an "existential threat" to the division of powers guaranteed by the Constitution.

Prime Minister Justin Trudeau, later in the House of Commons, said 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 Impact Assessment Act over what the province argued was its overreach into provincial powers.

The act, previously known as Bill C-69, was given royal assent in 2019. It 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 had 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 the 204-page document released Tuesday sided with Alberta, calling it a "breathtaking pre-emption of provincial authority."

All five justices agreed the threat of climate change must be addressed, but the majority opinion said the environment is not the sole jurisdiction of the federal government, so it doesn't have unilateral power to regulate it.

"Intra-provincial activities are not immune from federal government regulation, providing that regulation remains within the constitutional dividing lines," Chief Justice Catherine Fraser wrote in the opinion released Tuesday.

It adds 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 (of) environmental protection regimes among multiple jurisdictions."

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."

Alberta Premier Jason Kenney tweeted that the court's opinion represents "an historic victory, and central part of our strategy to fight for a fair deal!"

His minister of energy, Sonya Savage, said on Twitter that it's a very good day for Alberta and the energy sector.

"The Ottawa-knows-best attitude of the Trudeau government is an attack on Alberta's right to control its own natural resources — one Peter Lougheed fought hard for in the 1980s," she said.

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.

Joshua Ginsberg with Ecojustice, an environmental law charity, said that while the court ruling is disappointing, his organization would support the appeal process.

"The [law] improves upon previous iterations of environmental assessment legislation, which failed to provide a credible process for decision-making on industrial projects with serious environmental impacts," said Ginsberg in a statement.

"Ecojustice remains committed to standing up to the likes of the Alberta government, including going to court to defend laws that protect our climate, biodiversity and a healthy environment for everyone in Canada."

"We are very confident that this is constitutional, that our position will be upheld," said Federal Natural Resources Minister Jonathan Wilkinson before the start of Question Period.

"But what I would say to you is the whole point of an environmental assessment process is to have rigour, to ensure that we actually are addressing substantive environmental concerns at the very early stages, such that good projects can go ahead and projects that actually are not able to be conformed to good environmental standards do not."

The federal minister of justice and the federal minister of the environment, David Lametti and Steven Guilbeault, also issued a joint statement on the opinion, emphasizing that the opinion does not have the effect of striking down the legislation.

"It is important to note that the decision of the Alberta Court of Appeal is advisory in nature and the Impact Assessment Act and regulations remain in force," they said.

Calgary Conservative MP Michelle Rempel said in a release that the Court of Appeal's opinion "confirms what Albertans already knew: Bill C-69 was nothing more than an unconstitutional, ideologically driven power grab by [Prime Minister] Justin Trudeau that aimed to destroy Alberta's energy sector."

"This government's constant attacks on Alberta's economy has cost thousands of jobs and destroyed livelihoods. After six years of ideological attacks by Justin Trudeau, Alberta deserves recompense for the destructive policy decisions."

The Canadian Association of Petroleum Producers, the oil and gas industry's largest lobby group, said the decision keeps the necessary constitutional balance between provincial and federal governments "in making decisions regarding projects that will advance Canada's prosperity while protecting our environment."