A US federal court in San Francisco on Friday denied an appeal by Monsanto in the cancer trial over its Roundup weedkiller and upheld an award of $25 million in damages.

It was the latest setback for Monsanto's parent, German chemical giant Bayer, in its campaign to put an end more than 13,000 US lawsuits over the chemical.

The three-judge panel affirmed the district court's judgment in favor of Edwin Hardeman, who blamed the chemical in Roundup for causing his non-Hodgkin's lymphoma.

The ruling found the district court properly denied Monsanto's appeal "because evidence showed the carcinogenic risk of glyphosate was knowable at the time of Hardeman's exposure."

A jury originally ordered the company to pay $75 million but a judge later reduced that amount.

The ruling Friday said the award was "at the outer limits of constitutional propriety" but was acceptable, "Considering the evidence of Monsanto's reprehensibility."

Roundup weed killer is the subject of thousands of lawsuits in the US Roundup weed killer is the subject of thousands of lawsuits in the US Photo: AFP / Mark RALSTON

Hardeman said he used Roundup extensively on his land in Sonoma County -- north of San Francisco -- from the 1980s until 2012.

He filed a complaint against Monsanto in early 2016, a year after being diagnosed with cancer.

The case was considered a "bellwether" in the litigation against Monsanto, but the judges cautioned that "different Roundup cases may present different considerations, leading to different results."

"We are disappointed with the court's decision as the verdict in this case is not supported by the evidence at trial or the law," Bayer said in a statement, adding it would consider appealing to the US Supreme Court.

The agrochemicals and drugs giant has been plagued by legal woes since it bought Monsanto in 2018.

Bayer, which is not admitting any wrongdoing, maintains that scientific studies and regulatory approvals show Roundup's main ingredient glyphosate is safe.

The company set aside some $11 billion to deal with a wave of US lawsuits, and in February said it had settled some 90,000 of the cases.