Glyphosate is the primary ingredient in Roundup weed killer, popular for use in agriculture as well as for homeowners and in the landscaping industry. Citing new findings by the International Agency for Research on Cancer (IARC), state health officials added glyphosate to their list of potential carcinogens in July 2017 under Proposition 65, a 1986 initiative that requires warnings of exposure to products that pose a risk of cancer or reproductive harm.

Monsanto, backed by farming groups, challenged the listing in both California state and federal courts. In February, a federal judge in Sacramento blocked the state from requiring Monsanto to put a warning label on glyphosate products, an action that had been scheduled to take effect in July.

U.S. District Judge William Shubb said the international agency’s findings had been contradicted by other studies, including one by the U.S. Environmental Protection Agency that found no link to cancer. It is speculated among environmental groups that this judge came into the case with prejudice and bias based on the wording in his comments in the decision.

Thursday’s ruling involved a separate issue, Monsanto’s claim that the state was illegally delegating lawmaking powers to an unaccountable foreign agency, the International Agency for Research on Cancer. In rejecting that argument, the Fifth District Court of Appeal in Fresno preserved California’s authority to list glyphosate as a possible carcinogen and prohibit discharge of the chemical into waterways.

The voters approved Prop. 65, which named the international agency, a cancer research arm of the World Health Organization, as the body to identify cancer-causing substances, Presiding Justice Brad Hill said in the court’s 3-0 ruling. He noted that the U.S. and 24 other nations belong to a council that governs the agency.

The agency’s “reputation and authority on the world stage — and relatedly its funding — is dependent, in part, on its work being accepted as scientifically sound,” Hill said. He said the state, through its voters, had exercised lawmaking powers to require warnings on potentially dangerous products, and legally left factual decisions to an internationally supervised body.

Monsanto could ask the state Supreme Court to review the ruling. A company vice president, Scott Partridge, said in a statement that “no regulatory body in the world has concluded that glyphosate causes cancer,” and that independent reports have found that the international agency had used “flawed and incomplete science” to reach a contrary conclusion.

Environmental advocates hailed the ruling.

“This is a win for science and democracy,” said Rebecca Riley, a lawyer for the Natural Resources Defense Council in San Francisco. “The ruling clearly backs the voters’ choice to rely on expert scientific bodies to add dangerous chemicals to its list.”

The ruling is “an existential threat to glyphosate, as it should be,” said Adam Keats, a lawyer for the Center for Food Safety.