TORONTO -- An Ontario law forcing gas stations to display stickers showing the cost of federal carbon pricing is illegal and should be thrown out, a new lawsuit asserts.

The unproven lawsuit from the Canadian Civil Liberties Association says the Federal Carbon Tax Transparency Act -- dubbed the Sticker Act -- violates free speech provisions of the Constitution.

"The sticker imposed by the Sticker Act constitutes compelled political speech," the lawsuit asserts. "Under threat of significant fines, it legislatively requires gas station owners to express the (government's) position."

The liberties group says in its filing it was unable to find a gas station owner willing to fight the law despite its "diligent attempts."

The Progressive Conservative government of Premier Doug Ford brought in the legislation as part of its failed legal battle with Ottawa over carbon pricing ahead of next month's federal election. The federal scheme imposes a charge in those provinces that don't have a carbon-pricing system of their own -- currently 4.4 cents a litre in Ontario.

Ford has consistently denounced the federal legislation as a "tax grab" and has said it wants consumers to know what the federal charge will cost Ontario drivers.

"We're going to stick it to the Liberals and remind the people of Ontario how much this job-killing, regressive carbon tax costs," Energy Minister Greg Rickford, who is named in the suit, told the legislature in April.

The association, however, says the stickers are part of Ontario's political campaign against Ottawa and there's no good reason to force anyone to display them.

"The Sticker Act requirements do not relate to any technical standards or any concerns about safety," the lawsuit states. "Comments Ontario has made about the Sticker Act in the Ontario legislature and to the public demonstrate that the content of the stickers are political in nature."

Rickford, who said the province was reviewing the lawsuit and planned to respond, defended the act.

"Ontario's government is standing up for the people by implementing transparency measures that reveal the hidden cost of the federal carbon tax on the price of gasoline," Rickford said in a statement.

Critics have complained the taxpayer-funded stickers are misleading because they fail, among other things, to reflect a federal rebate the Liberal government says will put more money into consumer pockets than they are paying at the pump for the carbon levy. It also fails to mention other taxes or gasoline costs.

The act forcing gas stations to put up the stickers took effect on Aug. 30. Failure to do so can carry a fine of $5,000 a day for a first offence, rising to $10,000 a day for subsequent offences. The government has said it would initially issue only warnings rather than fines.

Late last month, the Ontario government asked the Supreme Court of Canada to weigh in on its carbon-pricing battle with Ottawa. The province argues that Ontario's Court of Appeal was wrong to find the carbon price constitutional and within the federal government's right to impose.

Ford has said voters will have the ultimate say on the issue when they cast ballots federally next month.

Greenpeace Canada welcomed the challenge, saying the stickers are misleading, partisan and unconstitutional.

"A government forcing businesses to mislead the public about climate change is a worrying development," the organization's Keith Stewart said.