Royal Dutch Shell’s plans to drill in the Arctic this summer grew more uncertain Wednesday when a U.S. federal court rejected the environmental analysis underlying the lease of the offshore parcel.

The Interior Department’s Bureau of Ocean Energy Management used an “arbitrary and capricious” estimate that oil and gas production in the Chukchi Sea off Alaska’s northwest shores would yield 1 billion barrels of oil, the U.S. Court of Appeals for the Ninth Circuit said in its opinion. That analysis potentially understated the environmental risks of development, the court said.