Xcel Energy Inc. must remain on the sidelines of a legal battle that could decide whether the utility’s coal-burning power plant in Becker, Minn., must cut emissions to reduce haze over Voyageurs National Park.

U.S. Magistrate Judge Janie Mayeron on Wednesday denied Minneapolis-based Xcel’s request to intervene in a federal lawsuit by the National Parks Conservation Association and five other groups against the U.S. Environmental Protection Agency. The suit accuses the agency of not enforcing air pollution rules at Xcel’s Sherco power plant — a charge EPA has denied.

Mayeron said Xcel had no legal right to join the suit even though it could cost $280 million for new pollution controls. Xcel has denied allegations by the U.S. Interior Department that the plant’s emissions cause haze, or reduced visibility, over Voyageurs in northern Minnesota and Isle Royale National Park in Lake Superior.