WASHINGTON — A federal judge has thrown out a key section of an Interior Department rule concerning the threat to polar bears posed by global warming.

U.S. District Judge Emmet Sullivan ruled Monday that the Bush administration did not complete a required environmental review when it said the bear’s designation as threatened in 2008 could not be used as a backdoor way to control greenhouse gases blamed for global warming.

The Obama administration agreed with the Bush administration a year later, saying that activities outside of the bear’s habitat such as emissions from a power plant could not be controlled using the Endangered Species Act.

The Center for Biological Diversity, an environmental group that filed a lawsuit over the 2008 rule, said the decision puts the fate of the polar bear back in the hands of the Obama administration and Interior Secretary Ken Salazar.

“The Obama administration has the chance to do right by the polar bear,” said Kassie Siegel, an attorney for the group. “They need to decide whether the polar bear gets all the protections that other endangered species get, or whether they want to re-adopt a flawed Bush administration decision that exempts greenhouse gases” and other pollutants from the Endangered Species Act.

Sullivan’s decision directs the Interior Department to respond by Nov. 17 with a timetable for when it will complete the required environmental review. Sullivan left an interim 2008 designation intact while the case continues.

In a related ruling Monday, Sullivan upheld a ban by the U.S. Fish and Wildlife Service to ban imports of sport-hunted polar bears as trophies. Safari Club International and other U.S. hunting groups had sought permission to allow bear carcasses to be imported from Canada.

The Humane Society of the United States hailed the ruling, which it said rejected “the Orwellian claim that killing polar bears is somehow good for polar bears.”

Jonathan Lovvorn, the group’s senior vice president and chief counsel for animal protection litigation, said that just as the United States does not allow importation of tiger skins and baby seal fur, “American conservation law prevents American hunters from bringing home the heads and hides of imperiled polar bears shot in other countries.”

The Fish and Wildlife Service said in a statement that it is reviewing the decision to determine the agency’s next steps.

In June, Sullivan upheld a 2008 decision by the Fish and Wildlife Service to place the bear on the endangered species list as threatened because of melting sea ice.

Environmental groups had sued, saying the polar bear needed more protection under the Endangered Species Act. The state of Alaska and hunting groups argued that the listing was unnecessary.

Along with the listing, then-Interior Secretary Dirk Kempthorne created a “special rule” stating that the Endangered Species Act would not be used to set climate policy or limit greenhouse gas emissions, which contribute to global warming and melting ice in the Arctic Ocean.

It was that aspect of the rule that Sullivan set aside on Monday.

The polar bear is unique among species protected under the Endangered Species Act because it is the first to be designated as threatened because of global warming.