WASHINGTON — The Clinton administration moved Monday to ban the gasoline additive MTBE, an octane booster that has helped clean the air but has begun to contaminate water supplies around the country.

“The time has come to take action,” said Carol Browner, administrator of the Environmental Protection Agency. “Americans deserve both clean air and clean water, and never one at the expense of the other.”

MTBE, or methyl tertiary-butyl ether, is now used in one-third of the gasoline sold in the United States, primarily in areas with smog problems. The ban, expected to take up to three years to implement, is a “backstop measure” in case Congress can’t agree on a way to phase out MTBE, Browner said.

As many as 9,000 community water wells near leaking underground storage tanks in 31 states may be affected by contamination from MTBE, according to a study being published this week in the journal Environmental Science and Technology.

The administration also recommended moves by Congress that would curb the use of the additive while preserving a market for ethanol, a more expensive corn-based alternative to MTBE:

Eliminating a section of the 1990 Clean Air Act that led to widespread use of MTBE by requiring gasoline in areas with serious air pollution to contain at least 2 percent oxygen by weight.

Requiring the continued use of ethanol and other fuels made from biomass and other “renewable” sources. Such a requirement, opposed by the oil industry but backed by agricultural interests, could boost the price of corn as much as 10 cents a bushel while having a negligible impact on shoppers, according to the administration.

Browner and Agriculture Secretary Dan Glickman said the administration will insist on replacing the oxygenate mandate with some kind of renewable-fuels standard.

“The administration … is not going to take or support any action that undermines the continued growth of ethanol and other renewable fuels,” said Glickman, who appeared at a news conference with Browner.

Ethanol is a potent political issue in several Midwest corn-growing states, including Illinois, that could be pivotal in this year’s presidential campaign between Vice President Al Gore and Texas Gov. George W. Bush.

Reformulated, or oxygenated, gasoline is required to be sold in all or parts of California, Connecticut, Delaware, Illinois, Indiana, Kentucky, Massachusetts, Maryland, Missouri, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, Texas, Virginia and Wisconsin. Arizona also uses the fuel because its supplies primarily come from California refineries.

Ethanol is used instead of MTBE in Illinois and Wisconsin.

MTBE, which has been used in gasoline for two decades, has been detected in 20 percent of the groundwater in areas where reformulated gas is required, compared to 2 percent in other areas, according to the U.S. Geologic Survey.

California, which has more leeway than other states to regulate air pollution, has already decided to ban the use of MTBE by the end of 2002. California, which is seeking a waiver from the oxygenate mandate, estimated that substituting ethanol for MTBE would add 1 to 2 cents a gallon to the price of gas.

“This is great news. We are getting rid of a dangerous poison and taking a giant step forward for clean water,” said Sen. Barbara Boxer, D-Calif.

An EPA advisory panel recommended dramatically curtailed usage of MTBE because of potential water pollution problems and elimination of the oxygenate standard in the Clean Air Act.

The panel did not suggest, however, imposing the renewable-fuels requirement that the administration proposed Monday.

With advances in refining, neither MTBE nor ethanol is still needed to maintain air quality, the oil industry contends.

“We think it would be a mistake to substitute one mandate for another. That’s how we got into this in the first place, Congress legislating the content of gasoline,” said Edward Murphy, a spokesman for the American Petroleum Institute.

Eric Wohlschlegel, a spokesman for the Republican-controlled House Commerce Committee, said EPA’s ethanol proposal was “transparently political.” Browner “should spend more time on creating good public policy than on playing campaign manager to Vice President Al Gore,” he said.

Various bills are being introduced in Congress to phase out the use of MTBE while protecting the ethanol market. Sen. Tom Harkin, D-Iowa, said the administration is “taking the right steps in moving to get MTBE out of gasoline and putting more renewable fuels like ethanol into gasoline.”