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WASHINGTON — A federal judge late Friday delivered a significant setback to the Trump administration’s policy of promoting coal, ruling that the Interior Department acted illegally when it sought to lift an Obama-era moratorium on coal mining on public lands.

The decision, by Judge Brian Morris of the United States District Court of the District of Montana, does not reinstate President Barack Obama’s 2016 freeze on new coal mining leases on public lands. That policy was part of an effort by the Obama administration to curtail the burning of coal, a major producer of greenhouse gases contributing to climate change.

But the court ruling does say that the 2017 Trump administration policy, enacted by former Interior Secretary Ryan Zinke, to overturn Mr. Obama’s coal mining ban did not include adequate studies of the environmental effects of the mining, as required by the National Environmental Policy Act of 1970, or NEPA, one of the nation’s bedrock environmental laws.

“Federal Defendants’ decision not to initiate the NEPA process proves arbitrary and capricious,” Judge Morris wrote.