Several major U.S. oil refiners said Tuesday that they won't sell gasoline containing 15% ethanol despite recent government authorization for fuel makers to start distributing the fuel blend.

Valero Energy Corp., Marathon Oil Corp. and Tesoro Corp. said they would refuse to sell E15, a mix of 85% gasoline and 15% ethanol, at their gas stations, because it could harm older automobiles or void their warranties. They and most other refiners now sell a mix of 10% ethanol and 90% gasoline.

Their refusal could dampen the ethanol lobby's ability to convince the U.S. Environmental Protection Agency to go beyond its existing approval of the fuel blend for vehicles made since 2007. The EPA is currently weighing whether to expand its approval to older cars and light trucks despite opposition from auto makers and some environmentalists.

"While some government agencies may believe differently, Tesoro isn't convinced that E15 is ready for prime time," a Tesoro spokesman said.

The three companies own or license their names to a combined 12,700 gas stations in the U.S., about 7.8% of the total as of 2008, the latest figure available. Other refiners, including Exxon Mobil Corp. and BP PLC, declined to comment.

None of Valero's 1,000 wholly-owned retail gas stations will sell the fuel blend, while the 4,800 stations that license the Valero brand name could sell E15 only if they market the fuel under a name other than Valero's, said spokesman Bill Day. Valero is the largest independent U.S. refiner. Selling E15 is "not practically possible" because of the danger of drivers putting E15 into unapproved vehicles, Mr. Day said.

Although ethanol advocates cite research saying E15 won't damage vehicles, auto makers hold that E15 could harm car and light truck engines and void their warranties. The Alliance of Automobile Manufacturers, representing Ford Motor Co., General Motors Co., Toyota Motor Corp., and other auto companies, filed a petition with a U.S. appellate court in Washington on Monday challenging the EPA's approval for the sale of gasoline containing 15% ethanol.

"There's no warranty protection from engine and equipment manufacturers for E15," Mr. Day said. "We're not going to sell a product we can't guarantee."

To help meet the EPA's goal of having 12.6 billion gallons of ethanol blended into the U.S. fuel supply in 2011, Valero will instead install more E85 pumps to serve "flexible fuel" vehicles specifically designed to use fuel containing 85% ethanol, Mr. Day said. E85 is sold primarily in the Midwest.

Marathon Oil said it won't sell E15 at the 6,000 gas stations it either owns or licenses its name to until the company feels sure that the fuel won't harm engines, said company spokeswoman Angelia Graves. "Before a new product like this is brought onto the market, the research needs to be complete," Ms. Graves said.

The Renewable Fuel Association, an industry group representing ethanol makers, said that despite the EPA's approval of E15 its arrival at gas stations nationwide is still "a bit of waiting game."

"There is that concern for sure," RFA spokesman Matt Hartwig said in an e-mail. "I don't know of any [refiners] that have said no, but I don't know many that have said yes, either."