WASHINGTON (Reuters) - The U.S. Supreme Court on Monday turned aside the latest effort by a group of states led by Michigan to block Obama administration environmental regulations limiting power plant emissions of mercury and other toxic pollutants.

Spring flowers bloom in front of the U.S. Supreme Court in Washington March 29, 2016. REUTERS/Gary Cameron

The justices opted not to hear the states’ appeal of a December U.S. appeals court decision allowing the mercury rules to remain intact while the administration responded to last year’s Supreme Court ruling that the government should have considered the compliance costs when crafting the regulations.

Opponents of the regulations, which went into effect in April 2015 and affect mainly coal-fired power plants, have estimated they would cost $9.6 billion a year and raise electricity bills.

Monday’s action marked the second time this year the Supreme Court has spurned the states on the issue. Chief Justice John Roberts on March 3 declined their request for a stay to put the regulations on hold.

The Environmental Protection Agency has “blatantly refused” to follow the 2015 high court ruling, prompting the states to ask the Supreme Court to intervene again, said Andrea Bitely, a spokeswoman for Michigan Attorney General Bill Schuette, a Republican.

An EPA spokesman said the regulations “cut harmful pollution from power plants, saving thousands of lives each year and preventing heart and asthma attacks.”

Environmental groups that support the regulations backed the Supreme Court’s refusal to hear the case.

“The Supreme Court correctly rejected the latest industry challenge to these vital protections against dangerous, toxic pollutants,” said Sanjay Narayan, a lawyer with the Sierra Club.

The EPA has updated the regulations since last year’s ruling by the high court, finding in April that they were necessary even when costs that would be incurred by industry are taken into account.

The EPA’s April decision is itself challengeable in court. Coal company Murray Energy Corp has already filed a lawsuit.

The Supreme Court in June 2015 ruled that the Obama administration wrongly failed to consider compliance costs when it devised the regulations, which were intended to reduce deaths caused by air pollution and reduce cases of mercury poisoning that can cause developmental delays and abnormalities in children.

According to the EPA, the rule applies to about 1,400 electricity-generating units at 600 power plants.

The mercury rules are separate from Obama administration regulations intended to curb carbon dioxide emissions that the Supreme Court put on hold on Feb. 9 in a legal challenge brought by 27 states and others.

(Reporting by Lawrence Hurley; Editing by Will Dunham)

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