Shares in German chemicals and pharmaceuticals giant Bayer plunged as markets opened today.

The steep falls came after a second US jury ruled that blockbuster pesticide Roundup – made by recently-acquired Monsanto – causes cancer.

Bayer's stock toppled 10.5% in early trade, making it the worst performer on the DAX index of blue-chip German shares.

A US jury found that weedkiller Roundup was a "substantial factor" in the cancer of a US man who developed a lump in his throat after decades of spraying his garden.

This was the second major legal defeat to agrochemical giant Monsanto in a year.

Jurors had been asked to decide whether the ingredient glyphosate – the world's most widely-used herbicide that has long been linked to cancer by environmentalists – was a "substantial factor" in the man's condition. 

The trial now moves to phase two – deciding whether Monsanto's conduct makes it liable.

Monsanto, which has sold Roundup worldwide for more than 40 years, contends that scores of studies show the products are not dangerous if properly used. 

The case is the first in US federal court on the alleged cancer risk posed by Roundup, and will be seen as a test case, with thousands of other similar lawsuits already underway in the US. 

It follows the groundbreaking state trial last year brought by school groundskeeper Dewayne "Lee" Johnson. 

In August, California jurors unanimously found that Monsanto acted with "malice" and that its glyphosate weedkillers Roundup and Ranger Pro substantially contributed to Johnson's terminal illness. 

Monsanto was initially ordered to pay $289m to Johnson, who has two young sons and in the end stages of his cancer, before the damages were reduced to $78.5m. 

Germany's Bayer, which bought Monsanto last year, filed an appeal. 

Bayer expressed disappointment with the decision, but maintained that the science confirmed that glyphosate-based herbicides do not cause cancer.