The high court did not explain its decision, which left intact a $25 million ruling in favor of a California man who alleged he developed cancer after using the chemical for years.

The decision marks a major blow to the German conglomerate's legal fight against Roundup-related cases, and Bayer has set aside more than $15 billion to deal with a wave of US lawsuits linked to the weedkiller.

"Bayer respectfully disagrees with the Supreme Court's decision," the company said in a statement.

"The company believes that the decision undermines the ability of companies to rely on official actions taken by expert regulatory agencies," it added, referring to a 2020 federal finding that Roundup's active ingredient is not risky.

Sustainable Pulse and The Detox Project Director, Henry Rowlands, stated Tuesday; "This landmark decision by the Supreme Court leaves Bayer with nowhere to go – they now have to face the fact that one of their most famous products causes cancer, as well as causing widespread environmental damage."