PolyMet and the Department of Natural Resources (DNR) had petitioned the Supreme Court in February to overturn the lower court's January 13 ruling, which remanded the three permits to the DNR for a contested case hearing.

"The court of appeals' interpretation of the statute creates tremendous uncertainty for companies who want to invest in Minnesota and must seek permits from the DNR and Pollution Control Agency. We are looking forward to presenting our case to the Minnesota Supreme Court," said president and CEO Jon Cherry in a statement.

Earlier this week, the Minnesota Court of Appeals sent an air-emissions permit for the NorthMet copper-nickel mine back to state regulators for further review having ruled the Minnesota Pollution Control Agency did not adequately evaluate whether the air permit requested by PolyMet accurately reflected the size and scope of its proposed mine.

"We demonstrated through the extensive environmental review and permitting process that we can meet or exceed Minnesota's strict standards for nonferrous mines," PolyMet said in a statement.

A US$945 million Phase I development would create a 32,000 tonne per day operation which would produce about 1.2 billion pounds copper, 170Mlb nickel, 6.2Mlb cobalt and 1.6 million ounces of precious metals over 20 years with ore processed at a former steel plant.

Opponents to the project development celebrated another permit being sent back for review.

"The Court of Appeals decision today makes it even more clear: the process that granted permits for the PolyMet mine proposal is broken. It's clear that the permits that were issued to PolyMet did not protect human health and the environment, and it's time for our agencies to acknowledge and address that," said Kathryn Hoffman, CEO of the Minnesota Center for Environmental Advocacy, in a statement.

Shares in PolyMet Mining are trading at C20c, valuing the company at $206 million.