A federal appeals court on Thursday blocked Royal Dutch Shell from drilling oil wells off Alaska’s North Slope after finding that the Interior Department had failed to conduct an environmental study before issuing the company’s drilling permit.

In a long-awaited ruling, the court said that the Minerals Management Service, the federal agency in charge of offshore leasing, had violated the 1970 National Environmental Policy Act by failing to take a “hard look” at the impact that offshore drilling would have on bowhead whales in the Beaufort Sea as well as indigenous communities on the North Slope.

The decision canceled Shell’s permit to drill at a prospect called Sivulliq, about 16 miles off northern Alaska, and ordered the agency to begin the process from scratch.

“There remain substantial questions as to whether Shell’s plan may cause significant harm to the people and wildlife of the Beaufort Sea region,” the United States Court of Appeals for the Ninth Circuit, in San Francisco, said in its ruling. One judge, Carlos T. Bea, on the three-judge panel dissented.