RALEIGH, N.C. — A group critical of the merger that made Charlotte-based Duke Energy Corp. the country's largest electric company says it will seek to undo or revise the deal in a North Carolina appeals court.

NC WARN said Wednesday it is challenging the North Carolina Utilities Commission's approval of the deal combining Duke Energy with Raleigh-based Progress Energy.

The group says the commission's failure to fully protect consumers from costs to update and expand the company's nuclear reactors could overwhelm the promised merger-related savings of $650 million.

"In ignoring key merger costs, the N.C. Utilities Commission helped lock in Duke's business plan for serial rate hikes to build dirty and dangerous power plants North Carolina does not even need," NC WARN said in a statement.

Duke Energy says the group's bid to undo the six-month-old merger is meritless.

"Since the merger was consummated in July 2012, Duke Energy’s North Carolina and South Carolina customers have been benefiting from lower power plant fuel costs and a larger, combined fleet of power plants that ensures operational efficiency," spokesman Dave Scanzoni said in a statement.

The company plans to seek a rate increase within weeks. Duke Energy last year was allowed to raise electricity rates by 7 percent, an increase challenged by the state Justice Department.