Three environmental groups asked the state Thursday to put the brakes on just-issued permits for Minnesota's first copper-nickel mine, arguing that regulators should wait until a related legal challenge is completed.

Last week the state Department of Natural Resources (DNR) issued six crucial permits for PolyMet Mining Corp.'s $1 billion mine and processing plant, nearly completing one of the longest and most contentious environmental reviews in Minnesota history. Other air and water permits, from the state Pollution Control Agency, are awaiting approval by the U.S. Environmental Protection Agency, and are expected to be completed by the end of December.

But earlier this year some of Minnesota's leading environmental groups asked the state Court of Appeals to rule whether regulators should expand their environmental review because of signs that PolyMet envisions a much larger mine than it first proposed. The state should hold off on the permits until the court has ruled in that case, the environmental groups said.

In March, Toronto-based PolyMet released a financial document showing that the highest financial returns, and the ones most likely to attract investors, would come with a mine three times larger than the one described in its Minnesota permit. The 6,000-acre complex covered by the state permits would generate a rate of return of about 10 percent, far less than the 30 percent it originally anticipated, the document said. But a mine three times that size would generate an after-tax return of 24 percent.

It would also generate much more pollution, require more water treatment and increase the risks associated with the mine's tailings basin, the environmental groups said. "PolyMet's permits are based on a bait-and-switch," said Aaron Klemz, communications director for the Minnesota Center for Environmental Advocacy. "PolyMet didn't show their hand … it was too late for the public to comment."

The company and DNR officials have said the permits reflect the project that PolyMet has actually proposed, and that any expansion would have to go through another review and regulatory process.

"The final permits we received from the state last week … are based on a mine plan that was the subject of a 14-year-long environmental review and permitting process, and which we plan to build," said Bruce Richardson, company spokesman. "We have no other mine proposals."

Barb Naramore, assistant DNR commissioner, said the agency will give the groups' request careful consideration.

"But we have a great deal of confidence in the sound basis of our decision this summer to deny the request for [expanded] environmental review, and we have equal confidence in the permit decision last week," she said.

The three groups, Friends of the Boundary Waters Wilderness, the Minnesota Center for Environmental Advocacy and WaterLegacy, asked the DNR to decide on staying the permits by Dec. 3. If the DNR says no, they will seek an injunction from the Minnesota Court of Appeals, Klemz said.