WASHINGTON — The largest remaining source of uncontrolled toxic air pollution in the United States, the nation’s coal- and oil-fired power plants, will be forced to reduce their emissions or shut down under a federal regulation released today.

The long-overdue national standards for mercury and other toxic pollutants are the first to be applied to the nation’s oldest and dirtiest power plants.

About half of the 1,300 coal- and oil-fired units nationwide still lack modern pollution controls, despite the Environmental Protection Agency in 1990 getting the authority from Congress to control toxic air pollution from power plant smokestacks.

A decade later, in 2000, the agency concluded it was necessary to clamp down on the emissions to protect public health.

Decades of litigation and changing political winds have allowed power plants to keep running without addressing their full environmental and public health costs.

EPA administrator Lisa Jackson said in a statement that the standards “will protect millions of families and children from harmful and costly air pollution and provide the American people with health benefits that far outweigh the costs.”