A Hennepin County judge has cleared the way for the state to resume a long-standing lawsuit against 3M over pollution damage done to natural resources in the east metro area.

The lawsuit is potentially the largest of its kind brought by the state, and the largest in Minnesota since the U.S. Department of Justice sued Reserve Mining Co. in 1972 over its disposal of taconite tailings into Lake Superior.

The sticking point in the five-year-old lawsuit was 3M's attempt to disqualify the Covington and Burling law firm from representing the state of Minnesota.

The Washington, D.C.-based firm has represented 3M on regulatory and other legal matters related to perfluorochemicals (PFCs). The company argued that Covington might reveal confidential information to the state, resulting in an unfair advantage for the plaintiffs.

But District Judge John McShane, in an order issued Friday, denied the disqualification on grounds that 3M had waited too long to seek it — even though he agreed with 3M that the Covington firm violated rules of professional conduct by representing the state in a matter adverse to the company.

Minnesota Solicitor General Alan Gilbert said that McShane's ruling means the environmental case can proceed and hopefully "move quickly ahead" after the long impasse. Having Covington and Burling represent the state is essential to the suit, he said.

"These environmental cases are extremely complicated and damages can be very large as well," Gilbert said.

William A. Brewer III, attorney for the firm representing 3M, said the company had moved to disqualify Covington as soon as it became apparent that the firm had a conflict. "There was no intent by 3M to waive its right," Brewer said.

PFCs are a family of compounds made by 3M over 50 years, until 2002. They are still made by other companies and used in a vast array of consumer and manufactured goods.

The case, still potentially far from going to trial, is seen as a legal showdown over the issue of whether PFCs are harmful to human health.

The suit alleges that 3M agreed to begin a massive cleanup effort after PFCs were found in the Mississippi River near the company's Cottage Grove plant, and then in soil, surface water and groundwater around the company's four licensed disposal sites in Washington County.

Although the state and 3M reached accord on the cleanup, they were unable to settle the state's claim that it is entitled to compensation for damages to its natural resources caused by PFCs. That impasse triggered the state's lawsuit, filed in December 2010.

In his order, McShane ruled that 3M waived its right to seek disqualification of Covington because 16 months passed from the time the firm appeared on behalf of the state to the time that the company filed its motion seeking the firm's removal from the suit.

"This is an extraordinarily long delay, given the amount of pretrial preparation and discovery that was done during that period of time and the amount of time that would be required by a new law firm to assimilate that information," the judge wrote.

Brewer said he was pleased that McShane agreed that Covington had "violated its legal and ethical obligations to 3M" by representing the state. "The court's opinion enables 3M to protect its confidences shared with Covington from misuse," he said.