Washington ? A federal appeals court on Wednesday blocked some of the Bush administration’s changes to the Clean Air Act from going into effect, dealing a major setback to one of the White House’s biggest environmental decisions.

A three-judge panel of the U.S. Circuit Court of Appeals for the District of Columbia agreed with 12 states and several major cities that argued they would face irreparable harm to their environments and public health from the changes. The judges ordered the Environmental Protection Agency not to implement its rules change until the panel can make a final determination about the case.

That court challenge, which could last well into the next year, takes aim at an EPA rule making it easier for utilities, refineries and other industrial facilities to make repairs in the name of routine maintenance without installing additional pollution controls.

The court decision stops the new rule from taking effect, which would have happened later this week. The justices said the challengers had “demonstrated the irreparable harm and likelihood of success” of their case, the required grounds for stopping the rule.

But the judges also refused to reconsider an earlier decision not to block other EPA changes to the Clean Air Act, which gave coal-fired power plants and other industrial facilities in some states more flexibility to calculate pollution levels.

EPA made the maintenance rule final in October. It was first proposed in December 2002.

Suing to block the maintenance rule were the attorneys general of 12 states — Connecticut, Maine, Maryland, Massachusetts, New Hampshire, New Mexico, New Jersey, New York, Pennsylvania, Rhode Island, Vermont and Wisconsin — and officials from New York City, Washington, San Francisco, New Haven and a host of other cities in Connecticut.

Eliot Spitzer, New York’s attorney general, called it “a major decision.”

“When it comes to environmental policy, this court decision is as big a success as we’ve had in stopping the Bush administration from undercutting the Clean Air Act,” he said.

Tom Reilly, the Massachusetts attorney general, said the court “forced EPA to take back its early Christmas present to the coal-fired power plants in the Midwest.”

EPA spokeswoman Cynthia Bergman expressed disappointment with the ruling, but said it was still being reviewed by agency officials.

She said the maintenance rule was “intended to clarify the process for maintaining and operating a facility,” but that it emphatically “does not allow power plants to increase their emissions past their current Clean Air Act limits.”

“In fact, this rule will have little or no impact on emissions,” she said.

The White House, which had overseen the changes, referred all questions to the EPA.

Environmental and health groups, including Natural Resources Defense Council and the American Lung Association, also challenged the rule in the appeals court.

They argued EPA’s maintenance rule violates the Clean Air Act by letting power plants and other industries increase pollution significantly without adopting control measures, and public harm would result.

“The court agreed this rule would cause great harm to the public that could not be undone, and it’s likely the rule will be struck down for running afoul of the Clean Air Act,” said John Walke, NRDC’s clean air director.