If the law in question relates in substance to a federal head of power, that is “the end of the matter”, the province will not have legislative authority to enact the law.

In the present case the tensions between the British Columbia and Alberta governments was due to the proposed amendments to the Environmental Management Act, which would prohibit the “possession, charge or control” of heavy oil without first obtaining a Hazardous Substance Permit. British Columbia asserted that it has the power to regulate the ‘TMX’(Trans Mountain Expansion Project) in the environment’s interests & not exclusively, but to the extent that it may impose conditions on, or even prohibit the presence of heavy oil in, the province unless a hazardous substance permit is issued to the operator. The court of appeal disagreed, finding that the proposed legislation was not a valid exercise of provincial power, but was instead an impermissible regulation of a federal undertaking because it singled out the TMX pipeline, and had the potential to stop the entire operation of the TMX as an inter-provincial carrier and exporter of oil. The Court of Appeal stated the Supreme Court has clarified in recent years that the first task in determining the constitutional validity of legislation is to determine its “true character” or “dominant characteristic”. That determination is not to be conflated with deciding whether the law “impairs” a “vital part” of the federal jurisdiction over interprovincial undertakings. If the law relates in substance to a federal head of power, that is “the end of the matter” in view of the Court and hence it found it unnecessary to continue on to discussion of paramountcy or inter-jurisdictional immunity. For this, it reasoned ‘paramountcy applies where the validly enacted laws of two levels of government conflict or the purpose of the federal law is ‘frustrated’ by the operation of the provincial law. Where this occurs, the provincial law will be rendered inoperative to the extent necessary to eliminate the conflict or frustration of purpose’ & the inter-jurisdictional immunity ‘applies when a valid law of a province trenches upon, or impairs the “core” of, a matter under exclusive federal jurisdiction’

The court set out a structure by relying on the precedents Friends of the Oldman River Society v. Canada (Minister of Transport) (1992), in which the Court observed that “the environment” was a “constitutionally abstruse matter” that does not “comfortably fit within the existing division of powers without considerable overlap and uncertainty. When viewed in this manner it will be seen that in exercising their respective legislative powers, both levels of government may affect the environment, either by acting or not acting. The British Columbia Court of Appeal on May 24, 2019 unanimously held that the province of British Columbia does not have the constitutional authority to enact amendments to the regulation of an inter-provincial undertaking to the Environmental Management Act [Sections 91(29) and 92(10)(a) of Constitution Act, 1867].

It was an appeal from this judgment of the BC’s Court of Appeal that reached the Supreme Court and was heard on January 16, 2020. The Supreme Court unanimously dismissed the appeal and agreed with the decision and reasoning of the Court of Appeal for British Columbia. As a consequence, the apex court of the land acceded to the proposed legislation being unconstitutional by application of Doctrine of pith and substance.