DENVER — A federal appeals court on Friday upheld a rule prohibiting roads on nearly 50 million acres of land in national forests across the United States, a ruling hailed by environmentalists as one of the most significant in decades.

Mining and energy companies, however, say it could limit development of natural resources such as coal, oil and natural gas.

The 10th U.S. Circuit Court of Appeals backed the 2001 Roadless Area Conservation Rule after lawyers for the state of Wyoming and the Colorado Mining Association contended it was a violation of the law.