WASHINGTON — A glut of ethanol in the gasoline supply is threatening to push up prices at the pump and may have exacerbated the growing cost gap between regular gasoline and premium, some oil experts say.

Refiners have been trading so-called ethanol credits furiously in an effort to meet federal environmental mandates, helping to significantly push up the cost of those credits — a jump to more than $1 from a few pennies in the last several days, and drivers are feeling the effects, experts say.

Prices for premium gas are now about 30.2 cents over the price of regular, according to Trilby Lundberg of the Lundberg Survey. That is up from 24.1 cents in 2010 and 18.2 cents in 2000. Any increases could affect about a third of this year’s car models, because premium fuel is required or recommended for them, according to Edmunds.com.

Experts disagree on the reasons for a widening gap between the costs of regular and premium gas. Reasons for the ethanol surplus are even more broadly in dispute, between producers and the oil companies. Gas companies are required under federal law to blend a certain number of gallons of ethanol into the fuel. But refiners argue that some cannot reach that requirement because they are nearing or at the so-called blend wall, the maximum percentage of ethanol in gasoline that most gas stations can handle, 10 percent. They also note that is the maximum level recommended by auto manufacturers for most cars.