(Reuters) - A federal judge overseeing lawsuits alleging Bayer AG’s glyphosate-based weed killer causes cancer has issued a ruling that could severely restrict evidence that the plaintiffs consider crucial to their cases.

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U.S. District Judge Vince Chhabria in San Francisco in an order on Thursday granted Bayer unit Monsanto’s request to split an upcoming trial into two phases. The order initially bars lawyers for plaintiff Edwin Hardeman from introducing evidence that the company allegedly attempted to influence regulators and manipulate public opinion.

Thursday’s order applies to Hardeman’s case, which is scheduled to go to trial on Feb. 25, and two other so-called bellwether trials which will help determine the range of damages and define settlement options for the rest of the 620 Roundup cases before Chhabria.

But Hardeman’s lawyers contended that such evidence, including internal Monsanto documents, showed the company’s misconduct and were critical to California state court jury’s August 2018 decision to award $289 million in a similar case. The verdict sent Bayer shares tumbling though the award was later reduced to $78 million and is under appeal.

Under Chhabria’s order, evidence of Monsanto’s alleged misconduct would be allowed only if glyphosate was found to have caused Hardeman’s cancer and the trial proceeded to a second phase to determine Bayer’s liability.

Bayer denies allegations that glyphosate causes cancer, saying decades of independent studies have shown the world’s most widely used weed killer to be safe for human use.

But the company faces more than 9,300 U.S. lawsuits over Roundup’s safety in state and federal courts across the country.

Bayer in a statement welcomed Chhabria’s decision.

“The court’s decision to keep the focus of the trial on the extensive science relevant to human health is encouraging,” the company said.

Aimee Wagstaff, one of Hardeman’s lawyers, in a statement said she was confident the jurors will find Roundup caused the man’s cancer and proceed to the second phase.

Hardeman’s attorneys had opposed proposals to split up the trial on the grounds that their scientific evidence allegedly showing glyphosate causes cancer was inextricably linked to Monsanto’s alleged wrongful conduct.

Bayer has also asked that some of the plaintiffs’ evidence on causation, specifically a finding by the World Health Organization’s cancer unit that glyphosate is “probably carcinogenic,” be excluded in the first phase because it has no basis in science.

Chhabria, who has previously expressed skepticism of that finding, on Thursday said he would soon decide to which degree he would allow it to be introduced at trial.

The assessment is central to the plaintiffs’ claims, as other regulatory agencies, including the U.S. Environmental Protection Agency, have determined glyphosate likely does not cause cancer.