WASHINGTON • The Environmental Protection Agency is expected to approve today a tough new rule to limit emissions of mercury, arsenic and other toxins from the country's power plants, according to people with knowledge of the new standard.

Though mercury is a known neurotoxin profoundly harmful to children and pregnant women, the air-toxins rule has been more than 20 years in the making, repeatedly stymied by objections from coal-burning utilities about the cost of installing pollution-control equipment.

The new regulation is not expected to differ markedly in its rigorous emissions targets and timetable from a draft rule proposed by the EPA in March, said people briefed on the rule in broad terms. The rule follows several decisions by the administration of President Barack Obama to shelve environmental rules to mollify sharply critical business people. Among those decisions was a high-profile move last summer to halt new standards to cut smog.

Some analysts said the rule still could be delayed if it got caught up in the political negotiations to pass spending legislation. Still, if it lands as expected, the long-awaited rule governing toxins is sure to rile powerful utilities and their congressional allies who have lobbied the administration over the past few weeks to weaken or delay the standards.

The EPA and the administration declined to comment on the pending rule.

The fight to dilute the rule has centered on the amount of mercury that can be emitted, and the timetable to install pollution control equipment. In its draft rule from March, the EPA determined that the industry standard be 1.2 pounds of mercury per million BTUs of energy produced. Industry wants 1.4 pounds. But EPA arrived at its standard based on a formula set out under the Clean Air Act, and analysts said the agency could deviate from a formula established by law.

The act would give companies three years to clean up their emissions of mercury and about 70 other toxins, and utilities could appeal for at least one more year as they installed the necessary equipment. Much of industry has argued that the timetable is too tight and could lead to rolling blackouts. One group, the American Public Power Association, told the White House its members needed more than seven years to comply with the mercury rule.

The EPA estimates that by 2016, the proposed rules could avert between 6,800 to 17,000 premature deaths annually, a greater benefit than most other federal health and environmental rules are estimated to achieve.