Glyphosate, a herbicide widely marketed by Monsanto Co. and other companies, likely has the potential to cause cancer in humans, a World Health Organization agency said Friday.

The determination, published by researchers for the International Agency for Research on Cancer in a U.K. medical journal, is likely to fuel further debate over the safety of the heavily used agricultural chemical, which Monsanto MON, +0.32% sells under the Roundup brand.

Consumer and environmental groups have long warned of health problems that they say could arise from applying the weedkiller on farms, while agricultural companies have touted the product’s safety and environmental impact as preferable to other, harsher chemicals. Officials at Monsanto and agricultural-chemical trade groups contested Friday’s finding, saying decades of research had proved glyphosate’s safety.

Glyphosate is the most-used herbicide in the U.S., according to the Environmental Protection Agency. Farmers have ramped up its use over the past two decades with the advent of genetically modified crops, including corn and soybeans, which can withstand sprayings of the chemical. Herbicide-tolerant biotech plants were grown on 94% of U.S. soybean fields and 89% of U.S. corn fields last year, according to the U.S. Department of Agriculture.