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The US Supreme Court has blocked a key government attempt to limit pollution from the country's power plants.

In a 5-4 split, the court ruled that the Environmental Protection Agency failed to factor in the full financial cost to industry of the changes.

The government introduced new rules to restrict emissions of toxins, including mercury, three years ago.

But several US states and companies challenged the changes, and the issue now returns to the US Court of Appeals.

The government has made several attempts to strengthen the Clean Air Act, but the court said this latest move must include costs as well as health risks.

Justice Antonin Scalia, writing on behalf of the majority, said the EPA "must consider cost - including, most importantly, cost of compliance - before deciding whether regulation is appropriate and necessary".

The new rules began to take effect in April. The EPA said it was disappointed by the court's decision, but added that many companies have already invested in upgrading operations so that they complied with the latest provision.

Deaths

The court challenge was brought by industry groups and 21 Republican-led states. The objectors had argued that the cost of installing equipment to remove pollutants would have cost the power industry up to $9.6bn (£6.1bn) a year.

About 600 power plants are affected, most of which burn coal, with many in the South and upper Midwest. Among companies opposing the rule was Peabody Energy, the largest coal producer.

The EPA had argued that the benefits would have been much greater - between $37bn and $90bn annually - due to the prevention of thousands of deaths, illnesses and lost days off work.

Vickie Paton, general counsel of the Environmental Defense Fund, which backed the Obama administration, said the EPA should be able to address the concerns raised by the court because it has "already analysed the economics showing that the health benefits for our nation far outweigh the costs."