Monsanto Co. lost a court ruling that permits key witnesses to testify in a lawsuit claiming the company’s Roundup weed killer caused cancer for farmers and other users.

It’s a significant victory for the cancer victims, who need expert testimony to advance more than 300 suits in San Francisco federal court seeking to hold Monsanto liable for failing to warn consumers about the risk of using the weed killer. A jury trial hasn’t been scheduled, but the decision puts pressure on Monsanto to settle the cases.

“Moving forward, we will continue to defend these lawsuits with robust evidence that proves there is absolutely no connection between glyphosate and cancer,” Monsanto Vice President Scott Partridge said in an e-mailed statement. “ We have sympathy for anyone suffering from cancer, but the science clearly shows that glyphosate was not the cause.”

U.S. District Judge Vince Chhabria is evaluating the toxicity of the world’s most popular herbicide, the subject of a heated debate among scientists and regulators worldwide for more than 30 years.