The Supreme Court put on hold the linchpin of President Obama's climate policy, barring the Environmental Protection Agency on Tuesday from carrying out the administration's new Clean Power Plan to reduce greenhouse gas emissions from electric power plants.

It was a surprising decision of staggering proportions, with repercussions that go far beyond the U.S. electrical grid, threatening the credibility of the Paris Agreement on climate change reached by the world’s nations in December.

The Clean Power Plan, designed to reduce by nearly a third emissions from fossil fuel-burning electricity plants, is the central element of the pledge by the United States to reduce its greenhouse gas emissions by at least 26 percent by 2025.

It was an unusual intervention by the Supreme Court, given that a powerful appeals court had just weeks ago turned down a request by dozens of states and their allies in the fossil fuel industries to impose a stay on the new federal regulation.

By blocking enforcement of the rule, the justices sent a signal that conservatives on the court may be inclined to limit the agency's powers under the Clean Air Act.  The Supreme Court found in its 2007 decision Massachusetts v. EPA that the statute allows controls on carbon dioxide emissions that cause global warming.

It would have taken years for the Clean Power Plan to take full effect, but the first step would have been for states to file implementation plans starting in September. Planning was well under way for that. About half the states had joined in appealing the rule, and some of them had declared that they would have refused to file state plans. Now, none of them will have to meet the rule's deadlines, which the EPA will be powerless to enforce.

SCOTUSblog, an authoritative web site covering the Supreme Court, said that the order "will delay all parts of the plan, including all deadlines that would stretch on into 2030, until after the D.C. Circuit completes its review and the Supreme Court has finished, if the case does wind up there.  There appears to be little chance for those two stages of review to be over by the time President Obama’s term ends next January 20."

Josh Earnest, the White House spokesman, said "we remain confident that we will prevail on the merits." He said the EPA would continue working with those states that want to move ahead with pollution controls under the rule.

"I am extremely disappointed by the Supreme Court's decision," said Attorney General Kamala Harris of California, one of 17 states that argued in favor of the rule in the appeals court. "The Court's decision, and the special interests working to undermine this plan, threatens our environment, public health and economy."

West Virginia Attorney General Patrick Morrissey, whose state is the lead plaintiff challenging the rule, said "we are thrilled" by the "great victory."

But environmental advocacy groups said they were confident that the rule would eventually pass judicial muster, and that in the meantime the trend toward greener power would continue.

The U.S. Court of Appeals for the District of Columbia Circuit has scheduled arguments for June and is expected to rule by late summer or early fall. An appeal to the Supreme Court would most likely be decided next year, after President Obama is out of office.

"We are confident the courts will ultimately uphold the Clean Power Plan on its merits," said David Doniger of the Natural Resources Defense Council. "The electricity sector has embarked on an unstoppable shift from its high-pollution, dirty-fueled past to a safer, cleaner-powered future, and the stay cannot reverse that trend."