The Canadian government on Tuesday invoked a 1977 treaty in a U.S. court to save Enbridge Inc.’s disputed Line 5 oil pipeline from a demand by Michigan’s top officials to shut down service on Wednesday.

The 540,000 b/d system is scheduled to be shut down at the Straits of Mackinac crossing between Lake Michigan and Lake Huron under an order by Gov. Gretchen Whitmer. Whitmer and Michigan Attorney General Dana Nessel have lost state court battles for orders to stop Line 5 flows and cancel an agreement to bury the Straits pipe leg in a new $500 million utility tunnel.

“The United States undertook a solemn and reciprocal commitment to Canada…not to interfere with the operations of international hydrocarbon transit pipelines,” said the Canadian brief filed in the U.S. District Court for Western Michigan. “That obligation, with legally binding effect under international law, expressly applies to any measures instituted by a ‘public authority in the territory of either party’ — including the State of Michigan and this court…

“Canada has initiated discussions with the United States to resolve this matter without a pipeline shutdown. This court should prevent Michigan’s shutdown order from taking effect while treaty-related discussions…are ongoing.”

Canadian Natural Resources Minister Seamus O’Regan said the federal government worked with its Alberta, Saskatchewan, Ontario and Quebec counterparts on the U.S. legal move.

“Line 5 is a vital piece of infrastructure for Canada and the United States and has safely operated at the Straits of Mackinac for 68 years,” O’Regan said. “It remains the safest, most efficient way to transport fuel to refineries and markets and is a reliable source of energy for Michigan, Ohio, Pennsylvania, Ontario and Quebec.”

The court brief also countered claims by Whitmer and Nessel that the demand to close the Line 5 leg results from greater concern than Canada shows for the Great Lakes.

“Preventing pollution of those shared waters is an important public interest that Canada, the United States, and subnational jurisdictions have worked together to protect since at least the Boundary Waters Treaty of 1909,” said the Canadian government brief.

“There has not been a major pollution event involving the Mackinac Straits section of Line 5 in its 68-year history, and Canada understands that the federal agency responsible for regulating Line 5 as to safety,” the U.S.Pipeline and Hazardous Materials Safety Administration, “found no unsafe or hazardous conditions in reports based on monitoring and inline inspections of Line 5 in 2019 and 2020.”

The district court hearing was scheduled for Wednesday. Court-ordered mediation has been underway since April 16.

The Michigan Public Service Commission (MPSC) plans to open hearings next week (May 18) on the plan to bury Line 5 below the straits in a utility tunnel already state approved.