BILLINGS — Yellowstone National Park abruptly dropped plans to sharply reduce the number of snowmobiles allowed this winter, drawing a quick backlash from conservationists who want fewer of the machines.

Park officials said Monday that 720 snowmobiles would be allowed into the park daily beginning Dec. 15. That’s more than double the 318 proposed just two weeks ago.

The move marked the latest turn in a decades-long struggle between snowmobile advocates, who want more recreational access, and conservation groups that say too many of the machines degrade the park’s natural beauty.

The 720 figure matches the number of snowmobiles allowed for the past three winters in the park, which straddles the borders of Montana, Idaho and Wyoming. But the new cap appears to defy a September ruling by U.S. District Judge Andrew Sullivan in Washington, D.C.

‘‘They’ve totally done a U-turn,’’ said Amy McNamara with the Bozeman-based Greater Yellowstone Coalition. ‘‘It’s incredibly disappointing. They were en route to putting a decision in place that would have sent them in the right direction.’’

Judge Sullivan has said an earlier park proposal, to allow just 540 machines daily, would have caused air pollution and disturbed wildlife. He threw out that plan and told the park service to come up with a new one.

In response, park administrators said Nov. 3 that they planned to allow 318 snowmobiles daily while they worked on a long-term plan.

Monday’s sudden reversal of that decision followed a second federal judge’s ruling in a separate lawsuit.

District Judge Clarence Brimmer in Wyoming ruled Nov. 7 that the park could allow 720 snowmobiles daily, a figure in place since 2004, while officials come up with a long-term access plan.

Park spokesman Al Nash said administrators decided to revert to the old temporary plan so they could dive immediately into coming up with a long-term solution.

‘‘Our intent with the temporary plan was to be open. Complying with the judge’s ruling means we’ll be open. Therefore we have accomplished our intent by complying with the judge’s order,’’ Nash said. ‘‘We need to shift our focus to that long term.’’

Monday’s announcement was welcomed in Wyoming, where residents and many elected officials have fought bitterly to preserve snowmobile access. State officials have said up to 950 of the machines should be allowed daily.

‘‘Our view is the parks are there for the people to experience,’’ said Jay Jerde with the Wyoming Attorney General’s office. ‘‘They should be allowed into the park as long as that access doesn’t cause unacceptable impacts.’’

Yellowstone officials have said previously that it could take up to three years to come up with a long-term plan. Nash said that timeline is no longer so clear, but he declined to give a more likely schedule.