The Environmental Protection Agency is planning to offer generous time extensions -- reaching into the next century in some cases -- to metropolitan areas that fail to achieve federal air quality standards for ozone and carbon monoxide by the year-end deadline set by Congress.

A 300-page plan circulating in the agency would give states virtual autonomy to choose their own schedule and measures for achieving the standards, as long as they cut excessive levels of the toxic pollutants by at least 3 percent annually.

The plan is expected to encounter stiff opposition from members of Congress and environmentalists seeking to impose shorter deadlines and stringent pollution controls on the dozens of cities, including the District of Columbia, that are expected to fall short of the ozone and carbon monoxide standards by Dec. 31.

"We're trying to create a flexible approach," Paul Stolpman, director of policy analysis and review for the EPA's air program, said of the plan to be formally proposed later this fall. "Simply ordering the cities to do it has not shown a lot of success in the past."

Congress set standards for the pollutants in 1970 to be met five years later, then extended the deadline to this Dec. 31. Carbon monoxide, emitted chiefly from automotive exhausts, causes circulatory problems. While ozone high in the atmosphere helps to shield the earth from harmful ultraviolet radiation, ozone at low altitudes, formed by the mixture of sunlight and volatile organic compounds released from cars and factories, causes pulmonary and respiratory troubles.

At least 50 metropolitan areas are expected to violate the deadline requirement for either one of the pollutants. Last June, the EPA proposed construction bans for 14 areas that have not crafted sound plans to combat the pollutants.

The EPA and Congress agree on the need to extend the deadline for delinquent municipalities. But the EPA's blueprint for achieving the standards sharply contrasts with Clean Air Act reauthorization bills in both chambers.

According to the draft EPA plan circulated internally earlier this month, states will have two years to demonstrate how they will meet the standards in polluted cities. Their only requirement is to show how the offending cities will cut atmospheric levels of ozone and carbon monoxide by 3 percent a year.

"We will expect them to make as rapid a progress as feasible," Stolpman said. "But if it's Los Angeles and takes 15 years, it's 10 years for New York and it's five years in Atlanta, all of those plans would be acceptable to EPA."

Stolpman said such flexibility could mean that Los Angeles will take 25 years to meet the standard, but added that shorter deadlines and stringent pollution curbs would result in stopgap measures and not sound, long-term planning.

The EPA, he said, will impose the construction bans proposed in June, preventing new large plants that emit the pollutants. But the moratorium would be lifted, he said, for areas able to project achievement of the standards in five years.

The plan is more lenient than pending legislation in Congress that would set deadlines ranging from three to 15 years for delinquent cities while requiring tighter auto emission controls and new standards for certain industrial sources. The Senate bill would require alternative fuels and service station devices to trap gasoline fumes in the most polluted municipalities.

"What's being proposed is all carrots and no sticks for those cities in violation of the clean air standards," Sen. Timothy E. Wirth (D-Colo.), cosponsor of the Senate bill, said of the EPA plan.