PARIS, Nov. 22 — European Union environmental officials have determined that two kinds of genetically modified corn could harm butterflies, affect food chains and disturb life in rivers and streams, and they have proposed a ban on the sale of the seeds, which are made by DuPont Pioneer, Dow Agrosciences and Syngenta.

The preliminary decisions are circulating within the European Commission, which has the final say. Some officials there are skeptical of a ban that would upset the powerful biotechnology industry and could exacerbate tensions with important trading partners like the United States. The seeds are not available on the European market for cultivation.

In the decisions, the environment commissioner, Stavros Dimas, contends that the genetically modified corn, or maize could affect certain butterfly species, specifically the monarch, and other beneficial insects. For instance, research this year indicates that larvae of the monarch butterfly exposed to the genetically modified corn “behave differently than other larvae.”

In the decision concerning the corn seeds produced by Dow and Pioneer, Mr. Dimas calls “potential damage on the environment irreversible.” In the decision on Syngenta’s corn, he says that “the level of risk generated by the cultivation of this product for the environment is unacceptable.”