HICAGO, June 9 — Despite persistent concerns about genetically modified crops, they are spreading so rapidly that it has become almost impossible for consumers to avoid them, agriculture experts say.

More than 100 million acres of the world's most fertile farmland were planted with genetically modified crops last year, about 25 times as much as just four years earlier. Wind-blown pollen, commingled seeds and black-market plantings have further extended these products of biotechnology into the far corners of the global food supply — perhaps irreversibly, according to food experts.

"The genie is already out of the bottle," said Neil E. Harl, a professor of agriculture and economics at Iowa State University, speaking of genetically modified organisms, or G.M.O.'s. "If the policy tomorrow was that we were going to eradicate G.M.O.'s, this would be a very long process. It would take years if not decades to do that."

Most of the biotech fields are soybeans and corn planted in North and South America, the biggest food exporters. But biotech crops — genetically altered to do things like release their own insecticide or withstand the spraying of weed-killing chemicals — are being shipped or experimented with in many other countries, including China, India, Australia and South Africa.

They are even turning up where people least expect them: in countries where they are banned but a black market has developed; in food supplies where they are forbidden or shunned, like organic products; even in fields that farmers believe are completely free of genetically modified crops.

The rapid adoption and proliferation means that even as scientists and others debate the safety of altering foods' genetic codes to produce cheaper and bigger supplies, a large share of the world's population has little or no choice but to consume genetically modified crops.

One indication came last year when Starlink, a variety of genetically modified corn not approved for human consumption, accidentally entered the global food supply, leading to extensive food recalls in the United States and Japan over fears it could cause allergic reactions.

Starlink has not been shown to be harmful; indeed, there is little evidence that biotech foods are dangerous to humans. But the episode showed that seeds planted on less than 1 percent of America's corn acreage could easily spread from farm to farm, contaminate the nation's grain handling system and seep into global food supplies.

Seed companies, farmers, processors and food makers have spent more than $1 billion in the last six months trying to eradicate Starlink. But most experts agree that will take years.

In the meantime, experts say the spread of biotech crops creates an entirely new set of trade, regulatory and legal problems:

¶ Large countries with policies limiting the use of genetically modified crops may soon have to change course, because they will not be able to get enough nonbiotech crops to meet their import needs.

¶ Regulators are under pressure to develop new standards to determine what is and is not genetically modified — a situation complicated, as the Starlink episode demonstrated, by the commingling and cross- pollination of different crops.

¶ Big food and agriculture companies are facing legal and public relations challenges, because some farmers and consumers believe their products have been contaminated.

Gene-altered crops are already ubiquitous in the United States, where the Food and Drug Administration has deemed them "entirely safe." But Europe and parts of Asia remain wary of the crops, and there have been moves in those regions to halt or slow their import.

Skeptics say that tampering with nature could inadvertently alter species, harm wildlife and give rise to new problems, like herbicide-resistant "superweeds." They also worry about the long-term health consequences of eating foods that are armed with insecticides and foreign genes. And the critics suspect that the industry has intentionally flooded the world market with genetically altered seeds to pre-emptively settle the question of whether or not to adopt biotechnology

Opponents expected Starlink to be a turning point in the fight against genetically altered crops. But while the episode helped stall the advance of genetically modified wheat, potatoes and sugar, it seems to have served as proof, over all, of biotech's inexorable spread. Most food makers in the United States continue to use biotech crops, insisting they are safe and far too pervasive to avoid; meanwhile, relatively few American consumers seem to care.

Perhaps more important, the bulk of American grain sold for domestic and international use goes into animal feed, and thus far few farmers or big companies have opposed feeding biotech grain to livestock.

Indeed, biotech industry officials believe the game is nearly won. The United States, Brazil and Argentina account for about 90 percent of the world's corn and soybean exports. Bulk shipments from the United States and Argentina are predominantly biotech. And Brazil is widely believed to have a black market in biotech soybeans.

If Brazil legalizes biotech production, Europe and Asia — the world's two biggest purchasers of soy — would have almost nowhere to turn for adequate supplies of nonbiotech soybeans. Environmentalists in Brazil have protested biotechnology, and though the government there is split, industry officials in the United States say that Brazil is leaning toward allowing the use of genetically modified seeds.

"We are very hopeful that last domino will fall," said Bob Callanan, a spokesman for the American Soybean Association, a trade group that supports the use of gene-altered crops. "That's why the environmentalists are putting up a stink down there in Brazil. They know if that goes, it's all gone."

That would be a huge victory for biotechnology companies. Monsanto, Aventis, Syngenta and others have spent billions of dollars to create the crops, and some independent groups, including the United Nations, promote them as one answer to world health and hunger problems.

Andrew Cash, an analyst who follows the biotechnology industry at UBS Warburg, says that Europe already has little choice but to accept the crops, largely because Monsanto's Roundup Ready Soybeans, the primary biotech variety, are so widespread.

"Europe is learning its first lesson in the `beggars can't be choosers' world of agricultural reality — it's G.M.O. beans or no beans," Mr. Cash wrote last January.

Food companies are already having a hard time obtaining nongenetically modified crops. Grain handlers like Archer Daniels Midland and Cargill are charging extra to segregate and test crops to certify that they are nonbiotech.

And that is becoming harder to do. Some agriculture experts say that cross-pollination of biotech corn and seed corn, as well as poor and imperfect grain-handling practices, have thoroughly scrambled crops in a global food chain that for decades shipped bulk supplies of largely undifferentiated products.

Food makers around the world are finding traces of gene-altered crops in foods that were not supposed to be made with them; Midwestern farmers are complaining that wind is blowing pollen from gene-altered crops into neighboring fields planted with conventional corn.

Even organic crops labeled "G.M. Free" are testing positive for genetic modification. Organic growers are now considering a class-action lawsuit against the biotech industry that would seek damages for the contamination.

"We have found traces in corn that has been grown organically for 10 to 15 years," said Arran Stephens, president of Nature's Path Foods, an organic producer of breads and cereals based in Delta, British Columbia. "There's no wall high enough to keep that stuff contained."

Some critics of biotechnology see a sinister plot at work, with the industry ignoring the implications of widespread pollen flow and perhaps even encouraging a black market in biotech crops.

"They're hoping there's enough contamination so that it's a fait accompli," said Jeremy Rifkin, a longtime critic of biotechnology.

"But the liability will kill them," he said. "We're going to see lawsuits across the Farm Belt as conventional farmers and organic farmers find their product is contaminated."

The world's biggest biotech seed companies acknowledge that some pollen may go astray. And they acknowledge that they cannot guarantee that even the conventional seed they sell is 100 percent free of genetic modification.

Agriculture, they say, is prone to mishaps.

"By and large, where there are crops grown, and where G.M. materials are approved, the issue is with us," said Dean Oestreich, a vice president at Pioneer Hi-Bred, the world's largest seed company. "Our basic seed stocks are pure. But there's always adventitious presence, which means small amounts of unintentional presence through pollen flow and physical mixing."

Because of all this commingling, the companies are calling on regulators in many countries to relax tolerance standards for crops, to avoid trade, labeling and legal problems.

Zero tolerance, said Jeanne Romero-Severson, a professor of agriculture at Purdue University, is simply not realistic.

"If your standard is 100 percent pure," she said, "you better stop eating right now."