A US judge on Monday conditionally upheld a ruling against Monsanto which found the agro-chemical giant's weedkiller product Roundup was responsible for a 46-year-old gardener's terminal cancer.

Judge Suzanne Ramos Bolanos rejected Monsanto's appeal for a new trial, court documents showed, but significantly reduced the payout, originally set at 289 million dollars, to 78 million dollars.

Dewayne Johnson was awarded the payout by a jury in August, potentially setting a precedent for other cases against the company and representing a huge blow to Monsanto, owned by German conglomerate Bayer.

Bolanos ruled that should Johnson accept the reduced payment, Monsanto's motion for a new trial would be denied and gave him until December 7 to decide.

The California judge said last week she was considering reducing the 250-million-dollar punitive damages award as she did not believe the trial had clearly proved that Monsanto's product caused cancer and that it did not warn of the dangers and concealed them.

She changed her mind to agree with the original verdict on Monday, but reduced the payout.

Monsanto is currently the subject of thousands of other lawsuits in the US from people who claim to have been adversely affected by glyphosate.

Johnson was diagnosed with terminal lymphatic cancer in 2014 and his case was moved up to be heard ahead of others as his doctors said he did not have long to live.

His attorneys said that he had regularly sprayed Roundup, which contains the controversial compound glyphosate, and another Monsanto product, Ranger Pro, in his job with a San Francisco Bay Area school district.

Glyphosate is claimed to cause non-Hodgkin lymphoma, a type of cancer affecting white blood cells. In a 2015 study, the World Health Organization found glyphosate "probably" caused cancer.

Monsanto vigorously rejects the allegations and says the herbicide glyphosate is safe.

Monsanto and Bayer have pointed to "over 800 scientific studies, the US environmental authority EPA, the National Health Institute and observers worldwide," which the companies say have supported their conclusion that the ingredient is not carcinogenic.