Ontario's attorney general and environment minister are urging the federal government to not impose a "job-killing" carbon tax in their province, and warning they are already working on two court challenges to fight the potential tax.

Attorney General Caroline Mulroney and Environment Minister Rod Phillips were expected to announce details about Ontario's PC government cancelling the cap-and-trade program — the costs of which are still unclear — but instead used the news conference to attack the federal carbon tax that could replace it.

Phillips urged Liberal Prime Minister Justin Trudeau to scrap the plan outright, calling it a cash grab that would punish both businesses and families in the country's most populous province.

"Cancel your carbon tax," he said.

This isn't about optics for people who cannot afford another tax. - Caroline Mulroney, Ontario attorney general

Phillips vowed the province will do everything in its power to fight a federal carbon tax, which he said Ontarians voted "overwhelmingly" to reject.

He also noted Trudeau's recent decision to soften carbon-pricing plans after hearing concerns from Canadian industry officials that the tax would affect their ability to compete in the global market.

Phillips said that amounts to the federal government starting a "climb down" on its plans — an admission, he said, that "carbon taxes are not effective," especially when there's uncertainty in the market.

Ontario joins Saskatchewan's legal fight

Mulroney said the province is already working with Saskatchewan to fight the imposition of a carbon tax, and said a court challenge will be filed at the Ontario Court of Appeal as well.

The government is set to take the position that a federal carbon tax would be unconstitutional.

Reporters challenged Mulroney on whether her government could actually win a court battle with Ottawa, or if the legal battle is all about optics.

"This isn't about optics for people who cannot afford another tax," she said.

The PC Party's election platform budgeted $30 million for the court challenge. Mulroney said she doesn't think it will actually cost that much.

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