The three-year battle against a $333 million joint wind-power venture between First Wind and Emera took a new turn on Tuesday, when opponents to the deal filed a fresh challenge at the Maine Supreme Judicial Court.

A notice of the court appeal also was filed at the Maine Public Utilities Commission.

At issue is the relationship between power generators and regulated utilities under Maine’s electric industry restructuring law. The law was meant to separate the two functions. But opponents to the First Wind-Emera venture to build new wind farms say it violates the law and could allow utility Emera to favor First Wind over other generators, to the harm of consumers.

Boston-based First Wind is the state’s largest wind energy producer. Emera is a Nova Scotia company that owns the major electric utilities in eastern and northern Maine.

The suit is being filed by Houlton Water Co., a municipally-owned utility in Aroostook County. It is being joined by the Industrial Energy Consumer Group, which represents mills and other large power users.

The matter came before the court last year, after Houlton Water objected to the PUC’s approval of the joint venture in 2012, which included a number of conditions. The court vacated the PUC approval, and sent the issue back to the agency for re-examination. Two commissioners then concluded the financial conflict was adequately addressed, apparently clearing the way for the two companies to move forward.

But opponents didn’t agree, saying the law is clear in banning such relationships, regardless of conditions.

“It’s an absolute prohibition,” Alan Stone, a lawyer representing Houlton Water, said on Wednesday.

A spokesman for First Wind declined comment.