Two weeks ago, it approved the unrestricted cultivation of biotech alfalfa over the objections of some environmental groups and the organic food industry. Last week, it cleared biotech sugar beets for planting, with some restrictions. Both the alfalfa and beets have a gene making them tolerant of the herbicide Roundup.

With Syngenta’s corn, however, the opponents are not only the usual anti-biotechnology groups but also a powerful industry that is normally receptive to biotechnology. The millers’ association, which has led the opposition, represents 43 companies, including giants like General Mills, ConAgra Mills and ADM Milling.

The association said that Syngenta’s own data indicated that as little as one amylase corn kernel mixed with 10,000 conventional kernels could be enough to weaken the corn starch and disrupt food processing operations.

Another concern of some in the food industry is that if the amylase corn is found in food supplies it could lead to recalls or disrupt exports.

Syngenta says the amylase enzyme is not active when the kernel is intact. It is most active, the company said, at certain levels of temperature, acidity and moisture found in ethanol factories but rarely in factories that make corn starch, corn syrup or corn chips.

Syngenta also said the corn would be grown only in the vicinity of ethanol plants. Farmers would be under contract and have financial incentives to sell their output only to that plant. Other steps would be taken to limit cross-pollination or inadvertent mixing in grain elevators.

But food processors and environmental groups said that some spread was inevitable.

“This is StarLink all over again,” said Margaret Mellon of the Union of Concerned Scientists. She was referring to the situation in 2000 when a genetically modified corn approved only for animal use got into the human food supply, prompting huge recalls and disrupting American exports.