A coalition of environmentalists, oil refiners, grocery industry representatives and the makers of power equipment has vowed to fight the change, and the auto industry has also expressed concern.

In a letter to Growth Energy, an ethanol trade association that filed the request for a higher blend in March, the E.P.A. said that “our engineering assessment to date indicates that the robust fuel, engine and emissions control systems on newer vehicles (likely 2001 and newer model years) will likely be able to accommodate higher ethanol blends, such as E15.” But the agency added that it would wait for the Energy Department to test a dozen vehicles, a task that should be completed in May.

The decision drew mixed reviews. At Growth Energy, Wesley K. Clark, a retired general who is co-chairman of the organization, said the reply was “basically a positive answer,” and sent a signal to his industry to invest in more plants, including some advanced plants that would make ethanol from corn cobs, wood chips and other nonfood sources.

Congress ordered increased use of ethanol in energy bills that passed in 2005 and 2007, but the plan has not worked as backers had hoped. Millions of “flex fuel” cars are on the road that can use ethanol blends as high as 85 percent, but fewer than 2 percent of service stations sell blends above 10 percent. Adjusted for its lower energy content, E85, as the higher blend is known, is more expensive than gasoline, and motorists thus have little reason to use it.

The automakers’ trade association praised the E.P.A.’s decision to wait for more test data, but expressed some concern about the idea that gas stations would begin selling fuel that might be good for younger cars and bad for older ones. A system based on model year “would create a tremendous likelihood of consumer confusion and misfueling,” said Charles Territo, a spokesman for the Alliance of Automobile Manufacturers.