CHICAGO (Reuters) - Monsanto Co has won support from eleven U.S. states in its attempt to stop California from requiring cancer warnings on products containing glyphosate, ratcheting up a legal fight over the company’s popular weed killer.

Monsanto's Roundup weedkiller atomizers are displayed for sale at a garden shop near Brussels, Belgium November 27, 2017. REUTERS/Yves Herman

Missouri, home to Monsanto’s headquarters, along with other farm states including Iowa and Indiana, said in court documents on Tuesday that the warnings would be misleading because there is no definite link between glyphosate and cancer.

Midwest businesses would need to include warnings on glyphosate products if California requires them or stop selling such goods because they may end up in the Golden State, according to the states’ filing.

California added glyphosate, the main ingredient in Monsanto’s Roundup herbicide, to its list of cancer-causing chemicals in July 2017 and will require products containing the chemical to carry warnings by July 2018.

The state acted after the World Health Organization’s International Agency for Research on Cancer concluded in 2015 that glyphosate was “probably carcinogenic.”

“The mandate imposes confusing and potentially inconsistent obligations on non-resident businesses, creating a strong incentive to abandon glyphosate markets altogether,” the states’ filing said.

For more than 40 years, farmers have applied glyphosate to crops, most recently on soybeans that Monsanto genetically engineered to resist the herbicide. Roundup is also sprayed on residential lawns and golf courses.

The controversy in California is a headache for the company as it faces a crisis around another herbicide based on a chemical known as dicamba that has been linked to U.S. crop damage.

Monsanto, which is being acquired by Bayer AG for $63.5 billion, developed the dicamba-based product following an increase in weeds resistant to glyphosate.

The 11 states are supporting a federal lawsuit Monsanto, the National Association of Wheat Growers and other agricultural groups filed in November to stop the state from requiring glyphosate warnings.

Monsanto said Wednesday it had discussed California’s mandate with officials in agricultural states as it proceeded with the lawsuit.

California’s Office of Environmental Health Hazard Assessment (OEHHA), which is named in the lawsuit, declined to comment. The office previously said it stands by the decision to include glyphosate on the state’s list of products known to cause cancer, as required under a rule known as Proposition 65.

“Proposition 65 is 30 years old and for every one of those years there have been strenuous attempts to kill it on the ground that it’s different from other states,” said David Roe, the rule’s primary author.

“They’ve always failed.”