(New throughout, adds Monsanto statement and background)
June 26 (Reuters) - Glyphosate, an herbicide and the active ingredient in Monsanto Co's popular Roundup weed killer, will be added to California's list of chemicals known to cause cancer effective July 7, the state's Office of Environmental Health Hazard Assessment said on Monday.
The agency said the designation under a state law known as Proposition 65 will proceed following an unsuccessful attempt by seeds and chemicals company Monsanto to block the listing in trial court.
Monsanto called the listing "unwarranted on the basis of science and the law." It has appealed the trial court's decision and vowed to continue its legal challenge.
"This is not the final step in the process, and it has no bearing on the merits of the case. We will continue to aggressively challenge this improper decision," Scott Partridge, Monsanto's vice president of global strategy, said in an emailed statement.
Monsanto has not been granted a stay so the listing will go ahead next month, the state environmental office said.
Listing glyphosate as a known carcinogen under California's Proposition 65 would require companies selling the chemical in the state to add warning labels to packaging. Warnings would also be required if glyphosate is being sprayed at levels deemed unsafe by regulators. (Reporting by Karl Plume in Chicago; Editing by David Gregorio)