California can require Monsanto to label its popular weed-killer Roundup as a possible cancer threat, a judge has ruled.

Monsanto and a group of citrus growers had fought the labelling proposal in California Superior Court, insisting Roundup poses no risk to people.

Roundup is widely used in agriculture throughout North America to clear land and control weeds.

The International Agency for Research on Cancer, a France-based health organization, concluded last year that glyphosate, the key ingredient in Roundup, is "probably carcinogenic to humans."

It based its findings on more than 1,000 studies from around the world, including studies of farmers and agricultural workers.

However, on Wednesday the European Chemical Agency concluded glyphosate should not be classified as a substance causing cancer.

Monsanto sued to block labelling requirement

Monsanto had sued California, the largest agricultural producing state in the U.S., in an effort to block a requirement for warning labels on the product.

The company had argued such a warning could hurt sales of the product, which it says has "low toxicity."

The chemical is not restricted by the U.S. Environmental Protection Agency nor by Health Canada. The EPA recommends agricultural workers stay out of fields treated with Roundup for 12 hours.

In her ruling last week, Superior Court Judge Kristi Culver Kapetan said none of Monsanto's objections to labelling for cancer risk were viable.

Health Canada reviewed the status of glyphosate in 2015 and concluded it could continue to be sold and used in Canada.

It does "not present unacceptable risks to human health or the environment when used according to the proposed label directions," the federal agency found.

However, it called for new risk reduction measures for use of the product.