Eleven organizations said they plan to sue over the wolves' removal in federal court in 60 days - the required first step for litigation under the Endangered Species Act.
Representatives of the groups say the estimated 1,500 wolves in Idaho, Montana and Wyoming are too few to ensure the species' survival - particularly given the states' plans to sponsor wolf hunts beginning this fall.
The government announced last week the predators had recovered from near-extermination last century. Management of wolves will now fall under state authority unless that decision is blocked in court.
State officials have pledged to keep wolves on the landscape. But they also would allow hunters and wildlife agents to kill hundreds of wolves, in part to reduce conflicts with livestock and big game.
"A lot of the killing may not be taking place just from hunters," said Michael Robinson with the Center for Biological Diversity, one of the groups that plans to sue. "It's going to be very systematic killing, with aerial killing and the trapping of wolves to put radio collars on them and then, after they return to their pack, killing the entire pack."
Sharon Rose, a spokesman for the U.S. Fish and Wildlife Service, a branch of Interior, said the challenge was expected.
The government lost an earlier federal court case over wolves in the northern Rockies, when it attempted in 2000 to reclassify them as threatened - a step down from endangered. Rose said her agency is confident its latest decision "will withstand scrutiny."
"Everybody took extra care to make sure that what we were doing was the right thing to do and that the population was actually ready to be de-listed," she said. "We believe the states will do a good job."
A spokesman for Wyoming Republican Sen. John Barrasso criticized the environmental groups for "interfering" in the management of the region's natural resources. Barrasso is one of many state leaders who had prodded the federal government to cede control over wolves.
"Most of these special interest groups wouldn't know a gray wolf if it walked up and bit them," Barrasso spokesman Greg Keeley said.
The Sierra Club, Defenders of Wildlife and The Humane Society of the United States also were among those that filed Wednesday's legal notice. They acted within hours of the government publishing its formal notice that the wolves will lose federal protection in 30 days.
The groups' attorney, Doug Honnold with Earthjustice, said he would ask for an emergency court injunction if the states move to kill wolves before the lawsuit is filed.
