Few industry debates are as heated these days as the one about labeling foods that contain genetically modified ingredients.

And while interest groups and advocates wage war in state legislatures, on ballots and in Congress over what should be disclosed on product labels, products certified as containing no genetically modified organisms are proliferating on grocery shelves without any nationwide mandatory regulations.

Moreover, many manufacturers are nodding to the public debate, adding the phrase "non-GMO" to their packaging without a verification process.

"We’ve put it on our labels because it was something our customers wanted to know," said Hitesh Hajarnavis, chief executive of Popcorn Indiana, which sells ready-to-eat popcorn.

So if more companies elect to put labels on their products stating that they are GMO-free, whether verified or not, does that make the fierce policy debate increasingly moot? "It’s an interesting question," said Jean Halloran, director of food policy initiatives at Consumers Union, which lobbies for mandatory labeling.

The shift toward voluntary labeling has also led to a lot of consumer confusion, as different labels, organizations and agencies issue seals or stamps that attest to compliance with few, if any, uniform standards. In addition, food companies are tacking the words non-GMO on items that would never be considered in need of such labeling.

The Non-GMO Project, the leading certification group in the United States, has verified more than 24,500 products, while the average grocery store contains 40,000 to 50,000 items, some of which are not food, according to the Food Marketing Institute.

Even more products have packaging that simply contains language stating that they are GMO-free. Boxes of the original Cheerios, for example, state "not made with genetically modified ingredients" on a side panel.

Nielsen, which conducts consumer research and analysis, said sales of non-GMO products exceeded $10 billion last year and grew at a faster pace than sales of gluten-free items over the last four years.

In a Nielsen study of 30,000 consumers published this month, 80 percent of respondents said they would pay more for foods with labels like "non-GMO" even though most of them do not necessarily trust food labels. And 61 percent of those consumers said it was "very" or "moderately" important to buy products with a non-GMO label, exceeded only by those saying it was important to buy products without high-fructose corn syrup.

Granted, the average store is unlikely to carry a full complement of the certified products, while food cooperatives, natural food stores and chains like Sprouts Farmers Market and Whole Foods Market have a higher proportion of items that have been officially certified.

Proponents of labeling note that while sales of products certified by the Non-GMO Project almost tripled last year, to more than $8.5 billion, that represents a small fraction of grocery stores’ total sales of $620 billion in 2013. "Unfortunately, we’re still a long way from the point at which voluntary labeling tackles the problem," said Halloran of Consumers Union.

Michael R. Gruber, vice president for federal affairs at the Grocery Manufacturers Association, a trade association representing major food manufacturers, said its members wanted the Food and Drug Administration to be the main regulator of food labeling, a role it has had historically. The industry spent more than $100 million last year to narrowly defeat various ballot initiatives to require more stringent labeling and it is backing a federal law that would pre-empt state laws on the issue.

"The political reality is that we are fighting the potential for a multistate patchwork of food-labeling laws and regulations for GMOs that would look very different from one state to the next," Gruber said. "We would like the FDA to provide the industry with guidance."

Yet in poll after poll, consumers have overwhelmingly said they want labels on foods that contain genetically modified ingredients. Most recently, 66 percent of respondents to an Associated Press-GfK poll last month said they wanted foods containing genetically modified ingredients to be labeled. Only 7 percent did not want such labeling.

Research by the Hartman Group found that 52 percent of consumers said they knew what genetically modified organisms were but less than a third could identify the crops that now are grown using genetically modified seeds.

"There’s no doubt that the industry is fighting a rear-guard action on this and trying to put it to rest," said Carl Jorgensen, director of global consumer strategy for wellness at Daymon Worldwide, a consumer research and consulting firm. "But there’s an aura of inevitability about it now."

Most corn, soy, canola and sugar beets, which are used to produce common food ingredients like high-fructose corn syrup, xanthan gum and ascorbic acid, are grown from genetically altered seeds, and papaya from Hawaii is largely genetically modified.

"It’s hard for the average consumer to remember, ‘Oh, I need to worry about corn and soy but I don’t have to worry about blueberries,’" said Jared Simon, who heads the snack foods business at the Hain Celestial Group, which owns brands like Arrowhead Mills and Earth’s Best. "There’s no easy way to navigate right now."

Hain Celestial recently redesigned its packaging for Terra Chips, and it added the phrase "non-GMO" out of concern that consumers seeking to avoid genetically altered ingredients might wonder about the canola oil used.

"Canola oil happens to be a high-risk ingredient in terms of GMO contamination," Simon said. "We felt it would be helpful to make it crystal clear."

The chips will eventually go through Non-GMO Project certification, but the backlog of products waiting for approval is getting longer and longer, said Courtney Pineau, the project’s associate director. "We’re still verifying on average about 600 products a month, but demand is intense," she said.

"We’re also having a lot of conversations with bigger, more conventional retailers, grocery stores, about how to get their private-label products verified," she said, although she declined to identify them. "That’s going to be a big trend this year."

Trader Joe’s already takes steps to ensure that its private-label inventory, which accounts for most of its sales, is free of genetically engineered products, and all products sold in Whole Foods are to be labeled by 2018.

If a traditional grocery chain like Kroger or Safeway were to begin labeling its private-label products, "that would be a game changer," Jorgensen of Daymon Worldwide said, noting that, unlike food manufacturers, grocery stores interact directly with consumers and thus see trends as they develop.

Other groups are springing up to check on well-known products. For example, a new advocacy group, GMO Free USA, is starting to have items like Kellogg’s Froot Loops cereal tested for GMOs and for the presence of glyphosate, an herbicide widely used on genetically modified crops like corn and soybeans.

Before the Non-GMO Project verified its first product in 2010, companies like Bob’s Red Mill Natural Foods and Clif Bar & Co. put their ingredients through what is known as identity verification to ensure they were not genetically altered.

Some of them are now putting products through the certification process because they think consumers are beginning to look for the project’s butterfly seal. Today, 32 of Bob’s Red Mill products carry the Non-GMO Project seal, and the company plans to have all its products verified.

Clif Bar & Co., however, worries that the Non-GMO Project seal has inadvertently created confusion among consumers about the meaning of another seal, the Department of Agriculture’s organic seal. The organic seal, governed by a federal law, is a guarantee that a product contains no genetically engineered ingredients — but many consumers do not know that.

"The organic seal is an assurance that food is grown without synthetic and toxic chemicals and in a manner good for the environment, as well as being non-GMO," said Matthew Dillon, senior manager for agriculture policy and programs at Clif. "While the intent of the Non-GMO Project is good, they have inadvertently caused more confusion in the marketplace."

About three-quarters of the ingredients Clif uses are organic, and the company’s goal is to be 100 percent organic. So it intends to stick with its current labeling practice, including the phrase "We source ingredients that are not genetically engineered" in the ingredients list on its packaging, Dillon said.