Supreme Court Chief Justice John Roberts Thursday quashed an attempt by states to roll back the Environmental Protection Agency's pollution standards for power plants.

The decision came a day after EPA filed a brief saying the states have no argument, and for the high court to move immediately to dismiss the request to stay the mercury and air pollution standards, called one of the most expensive rules for the electricity and coal sector.

The group of 20 states asked the court last month to halt the standards as the EPA works to finalize a cost assessment by mid-April. The Supreme Court last year said the EPA must assess the cost of its rules and conduct a study.

Much of the nation's power plant fleet has already complied with the regulations, costing them $9 billion annually. But a number of plants must still comply. A favorable ruling could have helped them escape making costly upgrades.