By Karl Plume

CHICAGO (Reuters) - Monsanto Co stepped up its defense of a widely used weed killer on Thursday by filing a lawsuit in California seeking to prevent glyphosate, the main ingredient in its Roundup herbicide, from being added to the state's list of known carcinogens.

The seed and agrochemicals company said it filed the suit against the state's Office of Environmental Health Hazard Assessment (OEHHA) and the agency's acting director, Lauren Zeise, in California state court, according to the filing seen by Reuters.

California law requires the state to keep a list of cancer-causing chemicals to inform residents of their risks.

OEHHA said in September that it planned to add glyphosate to the list after the World Health Organization's International Agency for Research on Cancer (IARC) classified it as a probable human carcinogen last March.

Monsanto has disputed assessment, citing decades of studies deeming glyphosate safe, including a 2007 study by OEHHA that concluded the chemical was unlikely to cause cancer.

"The IARC classification of glyphosate is inconsistent with the findings of regulatory bodies in the United States and around the world, and it is not a sound basis for any regulatory action," said Phil Miller, Monsanto's vice president of regulatory affairs.

Monsanto's lawsuit argues that listing glyphosate under Proposition 65, as the state's law is known, based on IARC's classification cedes regulatory authority to an "unelected, undemocratic, unaccountable, and foreign body" that is not subject to oversight by any state or federal entity.

Monsanto argues that the lack of oversight violates the company's right to procedural due process under California and U.S. law.

A listing would also require Monsanto and others offering products containing glyphosate to provide a "clear and reasonable warning" to consumers that the chemical is known to cause cancer, damaging Monsanto's reputation and violating its First Amendment rights, the company said.

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OEHHA did not comment, as it had not seen the lawsuit.

The case is Monsanto Company v. Office of Environmental Health Hazard Assessment, et al, case number 16-CECG-00183 in the Superior Court of the State of California, County of Fresno.

Roundup is used by farmers around the world, generating Monsanto $4.8 billion in fiscal 2015 revenue. Genetically modified seeds designed to tolerate glyphosate are immensely popular among corn and soybean growers.

But questions from environmentalists and other critics about the safety of the herbicide have dogged Monsanto for years.

Since IARC's classification last year, Monsanto has been named in numerous lawsuits accusing the company of knowing of the dangers of glyphosate for decades.

(Editing by David Gregorio and Dan Grebler)