A major association representing ethanol manufacturers is furious that the Environmental Protection Agency has delayed making a decision on whether to allow the ethanol content in gasoline to be increased to 15 percent, from 10 percent. But those worried that the increase will damage existing engines applauded the agency’s decision.

The request was made in March 2009 by Growth Energy, an ethanol lobbying group.

Originally a decision by the E.P.A. was expected last December. However, in December the agency said more study was needed and a decision would come in “mid-2010.”

In a statement released Thursday, the agency said all the necessary tests are not finished and a decision is not expected until this fall. In response, Growth Energy’s chief executive officer, Tom Buis, sent an angry letter to President Obama.

“As you would expect, we find this further delay unacceptable,” Mr. Buis wrote. “The fact that the federal agencies involved here cannot meet their own deadlines — on a decision that means so much to our nation — reinforces a public perception that government bureaucracy does not work in the best interests of the public. With fossil fuels getting dirtier, costlier and riskier to extract, as we are witnessing with the epic catastrophe in the Gulf of Mexico, now is the time we should move on expanding the production and consumption of clean, renewable fuels like ethanol.”

But associations representing engine-makers applauded the delay. Their concern has been that increasing the amount of ethanol will damage hundreds of millions of existing engines, ranging from chainsaws to automobiles.

Charles Territo, a spokesman for the Alliance of Automobile Manufacturers, said Friday that the group applauded the agency “for listening to our concerns and waiting until testing is complete before making a final decision. Ultimately it must be based on sound science, not political expediency.”

Kris Kiser, executive vice president of the Outdoor Power Equipment Institute, a trade group, has estimated that a change could damage small engines in everything from chainsaws to weed trimmers. Mr. Kiser said in an interview that even if the pumps were labeled E15, “We are very concerned about the wrong fuel getting into legacy products.”

But Growth Energy has told the E.P.A. that studies prove E15 will not damage engines and will result in cleaner air while reducing the nation’s reliance on oil.

Groups doing those studies included the Energy Department, the State of Minnesota, the Renewable Fuels Association, the Rochester Institute of Technology, the Minnesota Center for Automotive Research and Stockholm University in Sweden. But those assertions have been criticized by others, including Mr. Kiser, who say Growth Energy “cherry-picked” the data or misrepresented some statements.

For example, in one document Growth Energy mentions “Ford’s endorsement of blends up to E15.” While that statement might be interpreted as suggesting that Ford approves the use of E15 in its vehicles, last year a company spokeswoman, Jennifer Moore, said that was not true. She said Ford favored the use of biofuels but still had concerns about E15 because not enough research had been done.

Growth Energy has denied any effort to mislead people about the studies.