Ontario has filed its own constitutional challenge against the federal government over its planned carbon tax, broadening a legal battle that takes aim at Ottawa’s authority to implement a climate-change agenda.

Premier Doug Ford’s Progressive Conservatives had already backed Saskatchewan, which filed a challenge to the carbon tax in April.

On Thursday, Ontario urged Prime Minister Justin Trudeau to scrap the tax entirely after the federal government’s recent announcement that it was scaling back the scope of the environmental policy, which comes into effect next year. The Globe and Mail first reported on Wednesday that the federal government will tax most industries at about 20 per cent of their average carbon emissions, while sectors at high risk of foreign competition will only face a 10-per-cent rate.

“The Trudeau Liberals are finally confessing that the risk of a carbon tax is that it will invite an economic catastrophe,” Ontario Environment Minister Rod Phillips said Thursday morning as he unveiled the legal challenge with Attorney-General Caroline Mulroney. “Prime Minister, cancel your carbon tax.”

Mr. Phillips and Ms. Mulroney said Thursday that their government received a clear mandate from Ontarians to fight the carbon tax. Ontario’s move comes in the wake of the province scrapping its cap-and-trade program last month, which was brought in by the former Liberal government of Kathleen Wynne.

“Wherever the Trudeau Liberal carbon tax is being challenged in court, we want to be part of that fight,” Ms. Mulroney said. In addition to its own court action, Ontario has also joined Saskatchewan’s challenge of the federal carbon tax, which was filed in that province’s Court of Appeal. Ontario government lawyers will travel to the prairies to make their case there.

Saskatchewan Premier Scott Moe said on Thursday that he will review Mr. Ford’s constitutional challenge and that his province might join in Ontario’s case.

“It is our position that under the Canadian Constitution, provinces are sovereign in their assigned areas of jurisdiction. Therefore, the federal carbon tax is constitutionally illegitimate because it applies only in those provinces that have not exercised their own jurisdiction in a way that the federal government thinks they should,” Mr. Moe said in a statement.

Ms. Mulroney said that Ontario will spend up to $30-million to challenge the federal government’s carbon tax, starting with Thursday’s reference sent to the chief justice of the Ontario Court of Appeal.

“We will shortly be bringing a motion for directions to ask the court to determine the appropriate procedure for the reference,” deputy attorney-general Paul Boniferro wrote to the court. He attached a question approved by Mr. Ford’s cabinet, which asked simply whether the federal plan is “unconstitutional in whole or in part?”

Critics of Mr. Ford’s court challenge contend that it is a costly move that will have little chance of success. Manitoba’s Progressive Conservative government sought a legal opinion on federal carbon pricing, which concluded that Ottawa very likely has the constitutional authority to impose a carbon tax.

“As far as I know, the constitutional authority of the federal government is clear. Manitoba got an opinion on this and it is very clear the federal government is likely going to win. This is a waste of money. This is a pursuit, I think, to help the Conservatives in the next federal election,” said New Democratic legislator Peter Tabuns, speaking for the province’s Official Opposition.

Ms. Mulroney would not say whether she expects the province to win in court, but denied the assertion that the legal challenge was a symbolic gesture merely to appease Conservative voters. “Our position is that this tax is unconstitutional and outside of federal jurisdiction,” she said.

Federal Environment Minister Catherine McKenna said she was disappointed Mr. Ford’s government was choosing to spend millions of taxpayer dollars to fight the tax.

“Today, the Ford government made it absolutely clear that, like the federal Conservatives, it has no plan to tackle climate change,” she said in a statement. “Climate change doesn’t stop with a change in government. And in 2018, if you don’t have a climate plan, you don’t have a plan to grow the economy. Our kids and grandkids deserve better.”

The Globe and Mail, August 2, 2018