The state of Minnesota's private counsel in a longstanding ground water contamination dispute with 3M Co. was disqualified from the case Thursday.

Hennepin County District Judge Robert Blaeser ruled that the Washington, D.C., firm of Covington & Burling violated Minnesota rules of professional conduct by failing to fully disclose potential conflicts to 3M, which previously had been a client of the firm in matters that involved some of the same chemical issues.

In his ruling, Blaeser said Covington & Burling "exhibited a conscious disregard for its duties of confidentiality, candor, full disclosure and loyalty to 3M" for its past representation of the company on "issues at the heart of the state's case."

The law firm objected to the court's ruling and said late Thursday that "our client, the state of Minnesota, will be weighing its options" including a possible immediate appeal of the decision.

"We believe 3M failed to identify an actual conflict of interest and its attempt to disqualify the firm should, in any event, be barred because it came 15 months after the case was filed," the firm said in a statement.

The law firm representing 3M, Dallas-based Bickel & Brewer, called the decision "a resounding victory" for the big Maplewood-based manufacturer ''that underscores the importance of the ethical duties that are owed by lawyers to their clients."

The case involves the state's assertion that 3M contaminated river and groundwater sources in the east metro area through the release of perfluorochemicals (PFCs).

3M said Covington & Burling had a conflict of interest in representing the state in the case because the firm advised 3M on food packing issues involving PFCs in the 1990s.

Covington argued that its last representation of 3M was on an employee benefits matter in 2010. The law firm also said the state of Minnesota has been a Covington & Burling client on environmental matters since 1995.

In his ruling, Blaeser said there is "a strong inference that 3M's confidential information has been improperly accessed by Covington and will continue to."

Blaeser also criticized Covington & Burling for not requesting 3M's written consent to represent the state in the case.

"The obligation to discover, disclose and address potential conflicts of interest is, and remains on, Covington, not 3M," Blaeser said.

The judge said Attorney General Lori Swanson 180 days to find new counsel in the 3M case.

Swanson, through spokesman Ben Wogsland, said late Thursday that the office's lawyers "are reviewing the court's ruling."

David Phelps • 612-673-7269