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Tesoro Corporation, the largest oil refiner in the U.S. West, said it filed a lawsuit to block a California regulation which would raise the ethanol content in gasoline to as much as 10 percent by the end of 2009.

Tesoro's lawsuit against the California Air Resources Board calls for a review of the environmental and economic impact of the regulation, the San Antonio-based company reported in a Business Wire statement. It expects a ruling in 30 to 60 days.

Bruce Smith, Tesoro's chief executive officer, said in the statement that the review is needed because of emerging questions about how ethanol use affects the environment and food supply.

“We're not against ethanol in gasoline,'' said Sarah Simpson, a spokeswoman at Tesoro. ‘We're concerned about increasing the level to 10 percent and mandating it.'' Simpson said ethanol isn't as efficient as gasoline.

Some ethanol companies are counting on the increased demand in California, which leads the United States in gasoline consumption.

Pacific Ethanol Inc., a Sacramento-based ethanol producer, cited California's increased blending rule as a reason to expand output in the western United States.

The state “is going from 5.7 percent to 10 percent” of ethanol blended in gasoline, said Pacific Chief Executive Officer Neil Koehler in a September 29 interview. “We're still a drop in the bucket'' of supply to meet demand.