The supreme court on Wednesday sided with Michigan in ruling that the state’s lawsuit seeking to shut down a section of an ageing pipeline beneath a Great Lakes channel will stay in state court.

Justice Sonia Sotomayor wrote for a unanimous court that the Enbridge energy company waited too long to try to move the case to federal court.

The case is part of a messy legal dispute about a pipeline that has moved crude oil and natural gas liquids between Superior, Wisconsin, and Sarnia, Ontario, since 1953.

Dana Nessel, Michigan’s attorney general, sued in state court in June 2019 seeking to void the easement that allows Enbridge to operate a 4.5-mile (6.4km) section of pipeline under the straits of Mackinac, which link Lake Michigan and Lake Huron. Nessel, a Democrat, won a restraining order shutting down the pipeline from Ingham county judge James Jamo in June 2020, although Enbridge was allowed to continue operations after meeting safety requirements.

Enbridge moved the lawsuit into federal court in 2021, arguing it affects US and Canadian trade. But a three-judge panel from the sixth US circuit court of appeals sent the case back to Jamo in June 2024, finding that the company missed a 30-day deadline to change jurisdictions.

The pipeline at issue is called Line 5. Concerns over the section beneath the straits rupturing and causing a catastrophic spill have been growing since 2017, when Enbridge engineers revealed they had known about gaps in the section’s protective coating since 2014. A boat anchor damaged the section in 2018, intensifying fears of a spill.

The Michigan department of natural resources under Gretchen Whitmer, the state’s governor, revoked the straits easement for Line 5 in 2020. Enbridge filed a separate federal lawsuit challenging the revocation.

Enbridge won a ruling from a federal judge blocking the move, but Whitmer, a Democrat, has appealed to the sixth US circuit court of appeals. In March, the supreme court rejected Whitmer’s appeal claiming that she couldn’t be sued in federal court.

It was unclear how the federal ruling blocking Whitmer’s revocation attempt would affect Nessel’s case in state court. The company said in a statement that the judge in the Whitmer case had already decided federal regulators, not the state, are responsible for Line 5 safety and they had found no issues that would warrant shutting it down.

Enbridge also is seeking permits to encase the section of pipeline beneath the straits in a protective tunnel. The Michigan public service commission granted the relevant permits in 2023, but a coalition of environmental groups and Michigan tribes has filed a lawsuit seeking to void state permits for the tunnel. The state supreme court is weighing that case.

Enbridge also needs approval from the US army corps of engineers and the Michigan department of environment, Great Lakes and energy.

The pipeline is at the center of a separate legal dispute in Wisconsin as well. A federal judge in Madison last summer gave Enbridge three years to shut down part of Line 5 that runs across the Bad River Band of Lake Superior’s reservation. The company has appealed against the shutdown order to the seventh US circuit court of appeals, but it started work in February to reroute the line around the reservation.

The Bad River Band and environmental groups have filed a state lawsuit seeking to halt the work, arguing regulators have underestimated the damage the reroute construction will cause. That case also is pending.