The Environmental Protection Agency announced Wednesday a regulation more than two decades in the making that requires coal- and oil-fired power plants to control emissions of mercury and other poisons for the first time.

About 40 percent of the nation’s roughly 1,400 coal- and oil-fired utilities lack modern pollution controls on toxic emissions; the new requirement is expected to prompt the closure of some of the oldest and dirtiest plants.

Congress gave the EPA the authority to limit these toxins — which include mercury, arsenic, acid gas, nickel, selenium and cyanide — in 1990, but disagreements among federal regulators, industry officials and activists over how best to regulate them have stalled action until now.

The Washington Post reported the full details of the regulation on Friday, the day EPA Administrator Lisa P. Jackson signed the regulation into law, but the agency did not publicly disclose the rule until Wednesday.

“This is a giant victory for public health, especially the health of our children,” Jackson told reporters in a telephone call Wednesday, noting that she knew the full impact of polluted air, because as the mother of a son with asthma, “Fifteen years ago, my youngest son spent his first Christmas in a hospital, struggling to breathe.”

The EPA estimates the new regulation’s safeguards — which are slated to fully take effect in three years — will prevent as many as 11,000 premature deaths and 4,700 heart attacks a year by 2016 and will cost the industry $9.6 billion in compliance that year. By comparison, the agency projects reducing these emissions will save between $37 billion and $90 billion in 2016 in annual health costs and lost workdays.

The regulation could face legal and legislative hurdles, however: Some utilities have vowed to fight it in court, while the top Republican on the Senate Environment and Public Works Committee, James M. Inhofe (Okla.), said Wednesday he would seek to block it in Congress.

Some utility and coal industry officials said cleaning up these plants will cause severe economic hardship and could lead to power outages in regions of the country.

The rule will take effect in about 60 days. Along with the rule, the administration issued a presidential memorandum clarifying that an additional fourth year for compliance “should be broadly available to sources, consistent with the requirements” of the Clean Air Act. The memorandum notes that the EPA also has the ability to issue an administrative order for a fifth year, “should unusual circumstances arise that warrant such flexibility.”

Jackson estimated that only 4.7 gigawatts of the nation’s 1,000 gigawatts of electricity capacity, or less than one-half of 1 percent of the nation’s plants, would have to shut down as a result of the new standards.

Ann Weeks, who is senior counsel for the Clean Air Task Force and has sued the EPA over its failure to issue the mercury standards in the past, said, “Our work tells us this is feasible within the time constraints under the Clean Air Act, that it will not create electric reliability problems and that it will have very significant health benefits for Americans.”

But some industry officials said these requirements, along with other air pollution rules that the EPA has issued this year, will undermine the nation’s fossil-fuel sector.

“EPA has ignored the concerns of thousands of American workers and millions of consumers that rely on affordable and reliable coal-based electricity to power their factories and light their homes,” National Mining Association president and chief executive Hal Quinn said in a statement.

Joe Stanko, who heads the federal government relations team at Hunton & Williams and represents companies affected by the rule, said utilities would be unlikely to seek a fifth-year extension from the EPA, because they will still be exposed to citizen lawsuits from environmental groups.

“The proposed rule would close a significant number of power plants, and the changes in the final rule does not appear to alter that outcome,” Stanko said.

The standards are not likely to significantly affect two of the region’s major utilities, Constellation Energy and Dominion Power. Constellation has installed pollution controls on all of its facilities, while Dominion Power spokesman Jim Norvelle said the company doesn’t expect the rule to impact its operations beyond the measures it has undertaken to lower emissions.

Dominion will spend more than $3 billion by 2015 to install environmental controls at its coal-fired plants to comply with regulations, Norvelle wrote in an e-mail. The company is installing scrubbers at its coal-fired Chesterfield Power Station near Richmond and Brayton Point Power Station near Somerset, Mass., not far from Boston. Both are scheduled for a 2012 completion, Norvelle said.

In addition, Dominion this year announced that two power plants in Indiana and Massachusetts will close in summer 2014 and that two other plants in Virginia will close the following year.

“From what we have read, we appreciate that the EPA made certain adjustments, but we remain concerned about the challenges implementing this complicated rule in a short period of time,” Norvelle wrote.

Staff writer Darryl Fears contributed to this report.