The rule, issued by EPA in March 2005, aimed to slash power plant emissions of sulfur dioxide and nitrogen oxides by about 70 percent by 2015, preventing 18,000 deaths a year.

The US Circuit Court of Appeals for the District of Columbia found the EPA used a flawed approach in developing the CAIR rule, it could not be fixed in a piecemeal manner and the regulation had to be withdrawn.

EPA Administrator Stephen Johnson said he was "extremely disappointed" with the court's decision. He had yet to be briefed on the court's ruling and did not know what steps the agency would take next. When asked, he did not answer whether there was time left in the Bush administration to rewrite and fix the rule.

Frank O'Donnell, president of Clean Air Watch in Washington, described the court's ruling as "the legal equivalent of a dirty bomb: literally tens of thousands of Americans could see their lives cut short by dirty air."

Democratic Sen. Tom Carper, whose state of Delaware is affected by pollution from power plants in other states, said the court's ruling means Congress must take the lead in helping people who are forced to breathe smog coming from beyond their state's borders.

"We need to roll up our sleeves and finally pass legislative updates to the Clean Air Act so that Americans can breathe cleaner air," Carper said.

Carper and 12 other senators co-sponsored legislation last year that would reduce power power plant emissions of nitrogen oxide by 68 percent, sulfur dioxide by 82 percent and mercury by 90 percent by 2015.

"Since this administration has proved incapable of reading or following the law, Congress and the next president will have to write the standards that we need to protect the public health from air pollution," said Eric Schaeffer, director for the Environmental Integrity Project in Washington. (Editing by Christian Wiessner)