The Province of Ontario filed suit in the Ontario Court of Appeal seeking consideration of whether the Greenhouse Gas Pollution Pricing Act (GGPPA) is unconstitutional. The GGPPA became law in June 2018 and, as its name implies, allows the national government to set a price on greenhouse gas emissions which will be applied to provinces and territories that have not implemented a compliant program once the law goes into effect in January 2019. 

Ontario alleges that Parliament exceeded its constitutional authority in passing the GGPPA, arguing that the GGPPA is not authorized by the "national concern branch of the peace, order, and good government power" because the provinces are capable of regulating greenhouse gas emissions themselves and that "there is no need to expand the scope of federal jurisdiction to impose a one-size-fits-all federal carbon price." Ontario further argues that even if the GGPPA falls within the scope of a national concern, the GGPPA represents an unconstitutional tax because it does not provide an adequate nexus between the charges it imposes and its regulatory purpose. Multiple parties have intervened in the case, including a number of advocacy groups and the governments of Saskatchewan, New Brunswick, and British Columbia.