The European Union’s decision followed years of haggling and delay. Policymakers largely brushed aside the opinions of two of the bloc’s science agencies, which had found that glyphosate was not a carcinogen.

But glyphosate, which accounts for about a quarter of the global market, has been plunged into controversy since the International Agency for Research on Cancer, part of the World Health Organization, declared it a probable carcinogen in 2015. The finding, which has been disputed by a number of other government agencies, has made the weedkiller a magnet for controversy.

Glyphosate is also at the center of a federal case in the United States over claims that it causes cancer, and California has declared it a carcinogen, following in the footsteps of the international cancer agency.

Use of glyphosate has soared in the United States and other parts of the world over the last two decades, after Monsanto introduced crops that were genetically engineered to be resistant to the chemical. That meant key crops like corn, soybeans and cotton could be sprayed with the herbicide after they emerged from the ground. During that time, the presence of glyphosate in human urine increased 500 percent, according to a recent study by the University of California San Diego School of Medicine and published in the Journal of the American Medical Association.

Europe, by contrast, has largely shunned genetically modified crops, but glyphosate is still the Continent’s biggest seller. In Britain and Germany, it is used on as much as 40 percent of agricultural land, according to an industry trade group.

But political sentiment in Europe has been turning against Monsanto, the American company that has become the face of the agrochemical industry, even though it is in the process of being acquired by Bayer, a German chemicals giant. The European Parliament voted last month to ban glyphosate, a step that was nonbinding. And in September, the Parliament made Monsanto the first company to be barred from lobbying the chamber.

The science around glyphosate has become a muddle of allegations and counterallegations. Environmental activists have accused national regulators of hewing too closely to Monsanto’s wishes, while the industry has been exasperated that European politicians are overruling their science agencies. The litigation in the United States has only muddied the waters further, with evidence emerging that Monsanto ghostwrote both journalism and academic work, eroding trust in a company that had long been a lightning rod.