It’s sprayed on home gardens, Little League Baseball fields and, most heavily, the corn, soybean and other crops of the Central Valley.

More than 10 million pounds of glyphosate, the active ingredient in the weedkiller Roundup, are applied in California each year, according to government figures.

Now, after a yearlong legal battle, California’s environmental health agency has announced that it will list it as a known carcinogen.

The move would make the agency the first American regulatory body to do so. Yet the science is not settled, researchers say.

Two years ago, the cancer research arm of the World Health Organization deemed glyphosate a probable carcinogen. But other regulators have played down concerns of a cancer risk, including the U.S. Environmental Protection Agency which has said glyphosate poses “low toxicity for humans.”

The mixed science can be attributed in part to the way studies have been designed, said Nathan Donley, a senior scientist at the Center for Biological Diversity. “There’s a lot of controversy around it,” he said.

In court, the manufacturer of Roundup, Monsanto, argued that California had no basis to make the cancer designation and said it would hurt the company financially.

In January, a judge rejected the lawsuit. Monsanto, based in Missouri, has appealed.

“We disagree with the court’s ruling, and we will continue to fight the decision on the basis of sound science and the law,” the chemical giant said in an emailed statement.

The listing of glyphosate as a carcinogen would represent a warning to consumers, not a ban. Still, environmental leaders say they are counting on the move to tilt debates over its use.

A pivotal battleground would be in the Central Valley, where spraying of the herbicide is common and agricultural groups defend it as an essential tool in controlling weeds and safeguarding the food supply. (See a map of glyphosate use in California).

Along the coast, grass-roots groups opposed to glyphosate have already been securing victories.

Last month, school districts in both Burbank and Glendale agreed to halt their use of glyphosate after demands from parents.

Kathleen Hallal, a former PTA leader who led another successful effort to eliminate glyphosate use in Irvine, said she expected pressure on officials to intensify.

“A lot of parents have been awakened to the reality of Roundup being everywhere,” she said.