The Supreme Court of Canada found in a decision released on January 21, 2010 that the responsible authority cannot just conduct an environmental assessment screening study for a project listed on the Comprehensive Study List. In a unanimous decision of a 7-member panel in the case of MiningWatch Canada v. Minister of Fisheries and Oceans et al. the court held that the Canadian Environmental Assessment Act and regulations require that the environmental assessment track be determined according to the project as proposed; it is generally not open to a responsible authority to change that level. The responsible authority was free to use any and all federal-provincial coordination tools available, but it was still required to comply with the provisions of the Canadian Environmental Assessment Act pertaining to comprehensive studies. By conducting only a screening study, the responsible authority acted without statutory authority.
A link to the full decision is provided here.