* Shares up 4.6 pct after initial 6.4 pct gain
* Prospect of new trial, avoiding a lengthy appeal
* Tentative rulings often finalized with few changes
* Bayer stock still 16 pct below level prior to initial verdict (Adds shares, analyst comment)
SAN FRANCISCO/FRANKFURT, Oct 11 (Reuters) - The prospect of a fresh trial that could overturn $250 million damages against Bayer's Monsanto unit lifted the German company's shares, after an August ruling that it failed to warn users of the alleged cancer risks of its weedkillers.
The original verdict wiped 10 percent off the value of the company and marked the first such decision against Monsanto and its glyphosate-containing weedkillers Roundup and Ranger Pro.
Bayer, which bought Monsanto this year for $63 billion, faces more than 8,000 similar lawsuits in the United States.
On Wednesday, in San Francisco's Superior Court of California, Judge Suzanne Bolanos gave a tentative ruling to grant the company's motion for a new trial.
Such decisions are usually finalized after a second hearing with few major changes.
The case was brought by groundskeeper Dewayne Johnson and the August 10 verdict included an additional $39 million in compensatory damages. Johnson, whose doctors say he is likely to die within the next two years, attended Wednesday's hearing.
Bayer's shares jumped as much as 6.4 percent on Thursday, boosted by the prospect of the initial jury verdict being overturned or reduced without Bayer having to go through a more lengthy appeals process. The STOXX Europe 600 index, meanwhile, was down 1.7 percent.
"The presiding Judge Bolanos may indeed be inclined to deliver some better news, with yesterday's hearing indicating a willingness to allow a new trial altogether on the $250m punitive damages awarded in the trial," Barclays analysts said in a note.
The litigation has cast a pall over the stock, with Bayer shares still trading some 16 percent below the level prior to the original verdict.
Judge Bolanos, at the close of the two-hour hearing in front of a packed courtroom, said the parties had until Friday to make submissions in response to her ruling.
Bayer reiterated that the original jury verdict was "wholly at odds with over 40 years of real-world use, an extensive body of scientific data and analysis, including in-depth reviews by regulatory authorities."
In September 2017, the U.S. Environmental Protection Agency concluded a decades-long assessment of glyphosate risks and found that the chemical was not a likely carcinogen to humans. However, in 2015 the cancer unit of the World Health Organisation classified glyphosate as "probably carcinogenic to humans."
On Wednesday, the judge said that Johnson had failed to meet his burden of producing clear and convincing evidence of malice or oppression by Monsanto, a requirement for allowing a jury to award punitive damages.
Michael Miller, a lawyer for Johnson, told the court that the jury had reached a unanimous verdict after careful deliberations, supported by sufficient science.
"We have a jury that got it right," he added.
Monsanto had asked Bolanos in court filings on Sept. 18 to set aside the entire verdict or, in the alternative, reduce the award or grant a new trial.
Monsanto's lawyers said a comment made by Johnson's lawyers during the original trial that company executives would be "popping champagne bottles" if Johnson loses, were prejudicial and aimed at inciting jurors.
Johnson's case, filed in 2016, was fast-tracked for trial due to the severity of his non-Hodgkin's lymphoma, a cancer of the lymph system.
($1 = 0.8668 euros) (Reporting by Jim Christie in San Francisco Writing and additional reporting by Tina Bellon in New York; Editing by Bill Berkrot and Elaine Hardcastle)