The Michigan Supreme Court has rejected a permit for Enbridge's oil pipeline under the Great Lakes, dealing a setback to the Canadian operator's plan to keep crude moving through the Straits of Mackinac on the Line 5 system.

The decision closes off — at least for now — a regulatory route Enbridge had pursued to secure state authorization for the crossing beneath the Great Lakes, the segment of Line 5 that has drawn years of legal and political scrutiny in Michigan.

What does the ruling change?

The court's refusal to uphold the permit leaves the cross-border pipeline's future at the straits dependent on remaining legal and administrative avenues rather than a state-sanctioned approval. Enbridge has argued the tunnel project is the safest way to carry crude and natural gas liquids through the crossing; opponents, including state officials and environmental groups, have pushed to shut the existing dual lines down.

For refiners in Michigan, Ohio, Ontario and Quebec, Line 5 remains a supply artery. The line carries light crude and NGLs from Superior, Wisconsin, across Michigan's Upper and Lower Peninsulas and on to Sarnia, Ontario, feeding Midwest and Eastern Canadian refinery slates.

How did we get here?

Enbridge proposed relocating the aging pipeline segment that sits on the lakebed of the Straits of Mackinac into a bored tunnel beneath the straits, pairing the infrastructure plan with state permit applications. The company reached an agreement with the prior Michigan administration to pursue the tunnel, and litigation over both the tunnel permits and the state's later bid to revoke the easement for the existing lines has run through Michigan and federal courts since.

Tuesday's Supreme Court rejection removes the permit from the company's side of the ledger. The ruling reported by EnergyNow.com did not itself order a shutdown of the operating lines, but it narrows the procedural ground on which the replacement crossing stands.

What's at stake for supply?

Line 5 moves roughly 540,000 bpd of light crude and NGLs at full rates, and any forced outage would force rerouting by rail, truck and alternate pipelines. Refiners in Sarnia and Montreal have flagged the line as critical feedstock infrastructure, while Michigan officials have weighed the risk of a lakebed spill against the supply disruption a shutdown would cause.

Enbridge has consistently framed the tunnel as the durable fix: a bored crossing that would take the pipeline out of the open water and anchor it in rock beneath the straits.

What comes next?

Watch the follow-on filings. Enbridge can seek rehearing or pivot to federal claims it has raised in parallel litigation, and the state's broader effort to decommission the existing lines continues in the courts. The startup clock for any tunnel construction — already years away under the company's schedule — now depends on which of those tracks survives the next round of rulings.