An environmental research and advocacy group has found traces of a controversial herbicide in Cheerios, Quaker Oats and other breakfast foods that it says could increase cancer risk for children.

The report comes amid longstanding debate about the safety of the chemical glyphosate, which federal regulators maintain is not likely to cause cancer.

In its report, released Wednesday, the Environmental Working Group said that it tested 45 samples of breakfast foods made from oats grown in fields sprayed with herbicides. Then, using a strict standard the group developed, it found elevated levels of glyphosate in 31 of them.

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"There are levels above what we could consider safe in very popular breakfast foods," said Alexis Temkin, the group's toxicologist who helped with the analysis in the report.

The findings by the group, which has opposed the use of pesticides that may end up in food, were reported widely. But the question of whether glyphosate is safe is not so simple.

In fact, it is central to a raging international debate about the chemical that has spawned thousands of lawsuits, allegations of faulty research supporting and opposing the chemical and a vigorous defense of the herbicide from Monsanto, the company that helped develop it 40 years ago and helped turn it into the most popular weedkiller in the world.

Scott Partridge, a vice president at Monsanto, said in an interview on Wednesday that hundreds of studies had validated the safety of glyphosate and that it doesn't cause cancer. He called the Environmental Working Group an activist group.

"They have an agenda," he said. "They are fear mongering. They distort science."

Central to critiques of the glyphosate, which prevents plants from photosynthesizing, is a 2015 decision by the World Health Organization's International Agency for Research on Cancer to declare glyphosate a probable carcinogen.

That spurred a federal case in the United States over such claims and prompted California to declare it a chemical that is known to cause cancer.

Last week, a California jury found that Monsanto had failed to warn a school groundskeeper of the cancer risks posed by its weedkiller, Roundup, of which glyphosate is an active ingredient. The man's lawyers said he developed non-Hodgkin's lymphoma after using the weedkiller as part of his job as a pest control manager for a California county school system.

Monsanto was ordered to pay $289 million in damages. The company says it is facing more than 5,200 similar lawsuits.

Some research points to other potential health effects of glyphosate. In a study published last year in Scientific Reports, a journal from the publishers of Nature, rats that consumed very low doses of glyphosate each day showed early signs of fatty liver disease within three months, which worsened over time.

But many regulators and researchers say glyphosate is safe.

The classification by the International Agency for Research on Cancer has been disputed by United States and European regulators. And a recent major study, published by researchers at the National Institutes of Health, "observed no associations between glyphosate use and overall cancer risk."

In December 2017, the federal Environmental Protection Agency issued a draft human health risk assessment that said glyphosate was most likely not carcinogenic to humans.

The E.P.A. is currently reviewing public comments on that assessment as part of a standard review, and will decide on whether or not the agency needs any "mitigation measures" by 2019, a spokesman said Wednesday.

The United States Food and Drug Administration, which regulates domestic and imported food to make sure it does not exceed levels set by the E.P.A., said that based on 2016 samples, it had not found any violations of E.P.A. standards with glyphosate. More recent samples are still under review, an agency spokeswoman said.

The F.D.A. said Wednesday that it would consider the Environmental Working Group's findings.

Both Quaker Oats and General Mills, which makes Cheerios, said that their products were safe and met federal standards.

"While our products comply with all safety and regulatory requirements, we are happy to be part of the discussion and are interested in collaborating with industry peers, regulators and other interested parties on glyphosate," a Quaker spokesman said Wednesday.

A General Mills spokeswoman said, "Our products are safe and without question they meet regulatory safety levels."

Follow Mihir Zaveri on Twitter: @MihirZaveri.