WASHINGTON – The recent finding by an international panel that eating processed meat increases the risk of cancer could trigger warning labels under California law and a legal battle by meat producers and their trade groups to avoid the requirement.

California is one of the country’s largest producers and consumers of meat, and the meat industry is likely to fight any effort by the state to label its products under Proposition 65. That law, approved by the state’s voters three decades ago, requires warnings about products that contain substances known to cause cancer.

“I expect to see a lot of activity from the meat industry about that process,” said Patty Lovera, assistant director of Food and Water Watch, a consumer watchdog group. “They’ll beat it down in the court of public opinion.”

Industry advocates and legal experts anticipate that the meat producers will try to cast doubt on the findings or downplay their significance and could take California to court over the labeling requirements.

“We may have to,” said Janet Riley, senior vice president for public affairs at the North American Meat Institute, the industry’s trade association. “The level of reaction is not proportional to the level of threat.”

California’s Office of Environmental Health Hazard Assessment, the agency that enforces Proposition 65, relies heavily on the World Health Organization’s International Agency for Research on Cancer for guidance on what to list and label as carcinogenic. In California, the body’s research is considered as authoritative as that of the U.S. Environmental Protection Agency and the U.S. Food and Drug Administration.

The agency has added about 800 chemicals to the list since it was first published in 1987. Once a substance is listed, businesses with 10 or more employees have a year to comply with its labeling requirements.

Based on hundreds of studies, the international panel last month added bacon, sausage, ham, hot dogs and other processed meats to its list of Group I carcinogens, which include tobacco and asbestos. However, the panel emphasized that the classification of processed meats in that category did not mean the danger was equal to that of those substances.

The panel also classified red meat as “probably carcinogenic” based on “limited evidence” showing a relationship between its consumption and colon cancer. Red meat includes beef, pork, veal and lamb.

The panel also cautioned that the cancer risk from meat consumption was relatively low compared with smoking and excessive drinking, and it acknowledged that meat has known health benefits.

Critics of Proposition 65, which passed in 1986, have argued for years that its public benefits are limited because it gives consumers little context or comparison of relative risk. A warning typically only states that the product in question contains chemicals or substances known by the state to cause cancer.

“It doesn’t tell them what the risk is,” said Karyn Schmidt, senior director for regulatory and technical affairs at the American Chemistry Council, a trade group in Washington. “People don’t know how much they can eat.”

Groups that support Proposition 65, however, say products should be labeled so consumers can choose whether to accept the risk of exposure to carcinogens and if not, choose other products instead.

“The solution to that problem isn’t to label less,” said Stephanie Feldstein, population and sustainability director at the Center for Biological Diversity, an environmental group. “It’s to have better products.”

California’s environmental health office will now consider the World Health Organization’s findings. Allan Hirsch, the California agency’s chief deputy director, said the next step would be to formally notify the public of its intent to list processed meat and red meat as carcinogens, and a public comment period would follow.

Groups on both sides anticipate that the meat industry will attempt to discredit the science of the panel’s research in a bid to head off a listing. But it could be tough to dissuade the state agency from disregarding a report from an international body whose work it considers highly persuasive.

“It would be an uphill battle for the industry to contest this on the science,” said Laura MacCleery, regulatory affairs attorney for the Center for Science in the Public Interest.

Meat producers might have better luck arguing that federal law pre-empts Proposition 65. The U.S. Department of Agriculture regulates meat labeling, and courts have struck down state laws when they have come in conflict with federal requirements, though not always.

Bill Lockyer, who was California’s attorney general from 1999 to 2007, said he doesn’t think federal law would bar warning labels under Proposition 65.

“Pre-emption is regularly asserted, and usually rejected by a court,” he said. “For most areas like this, we have independent regulatory authority.”

In 2004, Lockyer sued manufacturers of canned tuna to require Proposition 65 labels warning of mercury in the fish. The industry argued that federal law pre-empted California’s, and lower courts initially agreed. A state appellate court ruled in 2009 that pre-emption didn’t apply, but it sided with the tuna producers for other reasons.

“I still, to this day, think we were right,” Lockyer said.

The court said that tuna didn’t have to be labeled under California law for two reasons: First, that the levels of mercury in the fish didn’t rise to the threshold that would trigger the labeling requirement, and second, that the mercury occurred naturally in the fish, rather than something added by the manufacturer.

Lockyer said those might be better arguments for meat producers.

Industry groups may also leverage any potential process on listing meat products under Proposition 65 to change a law they’ve often criticized. Efforts to reform the law have the support of Gov. Jerry Brown and Attorney General Kamala Harris.

Supporters of the law, however, said it protects consumers.

“I think consumers would benefit from having information on the package,” said MacCleery of the Center for Science in the Public Interest.