By Carey Gillam

Oct 20 (Reuters) - A plan by California environmental officials to list a commonly used herbicide as cancer-causing should be withdrawn, Monsanto Co told state regulators on Tuesday, saying California's actions could be considered illegal because they are not considering valid scientific evidence.

The formal comments were filed by Monsanto with the state's Office of Environmental Health Hazard Assessment (OEHHA), part of California's environmental regulatory office, on the final day the state accepted public comments about its intention to list glyphosate as a cause of cancer.

Glyphosate is the main ingredient in Monsanto's Roundup herbicide as well as many other herbicides.

The OEHHA gave notice in September that it intended to list glyphosate under Proposition 65, a state initiative enacted in 1986 to inform residents about cancer-causing chemicals. State officials said the action is required after the World Health Organization's (WHO) cancer research committee in March classified glyphosate as a probable human carcinogen.

The WHO's research unit said it reviewed many scientific studies, including two out of Sweden, one out of Canada and at least three in the United States before making its classification.

Roughly 8,000 comments were filed regarding the state action, according to officials, including those from Monsanto.

Listing glyphosate as a cancer cause "has the potential to deny farmers and public agencies the use of this highly effective herbicide," Monsanto said in its public filing. "Global regulatory authorities... agree that glyphosate is not carcinogenic."

Others applauded California's effort to list glyphosate as a cause of cancer. On Monday, several farming, public health and environmental groups sent a letter to OEHHA supporting the listing, and said that rising use of glyphosate presents a danger to people and animals.

Glyphosate was introduced in 1974 and is the key ingredient in Monsanto's branded Roundup line of herbicides, as well as hundreds of other products. It is registered in more than 160 countries, and has been embraced by farmers, homeowners and others because of its efficacy in killing tough weeds.

Story continues

But many scientific studies have raised questions about the health impacts of glyphosate and consumer and medical groups have expressed worries about glyphosate residues on food.

Since the WHO classification, the New York-based mass-tort firm of Weitz & Luxenberg and other firms representing U.S. farm workers have filed lawsuits against Monsanto, accusing the company of knowing of the dangers of glyphosate for decades.

Monsanto has said the claims are without merit.

(Reporting By Carey Gillam; Editing by Bernard Orr)