The battle over whether states can mandate that food containing genetically engineered crops must be labeled moves to the U.S. Senate on Wednesday.

The hearing in the Senate agriculture committee comes after the U.S. House of Representatives in July passed a measure that would block any mandatory GMO labeling by states and instead set a national voluntary standard.

The House bill potentially nullified a measure scheduled to take effect next year in Vermont, which would be the first such mandatory state labeling law. The food manufacturing industry is worried new laws will create consumer confusion and boost costs.

The hearing by the U.S. Senate Committee on Agriculture Nutrition & Forestry is expected to take testimony from five speakers, including Stonyfield Farm Inc Chairman Gary Hirshberg, a supporter of mandatory labeling, and Ronald Kleinman, a physician at the MassGeneral Hospital for Children who is an opponent of GMO labeling.

Last week, the Consumers Union and five other consumer organizations sent a letter to the Senate committee complaining that the lineup of speakers was not balanced and did not include a consumer representative. The groups said that "numerous polls have found that 90 percent of consumers favor mandatory labeling of genetically engineered food."

The Grocery Manufacturers Association (GMA), which represents more than 300 food companies opposed to mandatory GMO labeling, welcomed the hearing.

"We are confident that Congress will act on this issue this year given that members of both Houses and both parties have repeatedly told us that a 50-state patchwork of laws is not sustainable," GMA spokesman Brian Kennedy said in a statement.

The debate over the safety of GMOs heated up in March when the World Health Organization's cancer research unit classified glyphosate, the key herbicide sprayed on genetically modified crops, as probably cancer-causing for humans.

"This is really going to determine if they are going to go the path of the House... to figure out whether or not to keep consumers in the dark or if the tides are going to change," said Dana Perls, a food campaigner with Friends of the Earth, advocating for mandatory GMO labeling.