WHO: Weedkiller ingredient 'probably' causes cancer

But the agricultural chemical company that produces the herbicide has attacked the credibility of the WHO committee that assessed the chemical, accusing it of cherry picking the evidence.

The WHO's International Agency for Research on Cancer [IARC] reviewed the existing evidence on pesticides to establish a positive link between non-Hodgkin's lymphoma and glyphosate, which is widely used in agriculture and household herbicides, though it stopped short of naming the chemical a conclusive cause of cancer.

They found there was an increased risk of cancer among people who were exposed to glyphosate through their work, leading the agency to conclude that it was "probably carcinogenic to humans".

Co-author Lin Fritschi said regulatory guidelines should now be reviewed to limit or ban the use of herbicides containing glyphosate.

"There was sufficient evidence in animals that glyphosate is carcinogenic and evidence that it can cause some of the steps towards cancer in cells, but only a limited number of studies in humans so it was classified as a probable carcinogen," Professor Fritschi said.

There was no evidence that people who consumed crops that had been sprayed with herbicides containing glyphosate were at risk.

Glyphosate is used in more than 750 agricultural, household and forestry products and its use has increased since the development of genetically modified crops that are resistant to it.

Some of those crops have become the subject of high profile litigation in the Western Australian Supreme Court, where the seeds from one farmer's genetically modified canola crop blew onto his neighbour's organic farm.

But Monsanto, the chemical company that manufactures Roundup and developed the genetically modified "Roundup Ready" seeds that are resistent to its use, has accused the WHO agency of running "agenda driven bias".

Its Australian corporate affairs manager Adam Blight said 800 studies had demonstrated glyphosate's safety.

"What we're saying is that they've cherry-picked data," Mr Blight said.

"The IARC has not gone about a comprehensive review at all."

Monsanto has a controversial history including the production of Agent Orange, which was used to exterminate jungles during the Vietnam War and created highly toxic dioxin as a by-product during the 1960s, but has since reinvented itself as a sustainable agriculture company with an environmental conscience.

The agricultural chemical lobby group CropLife echoed Monsanto's criticisms, saying the IARC findings contradicted those of regulatory agencies around the world and would not affect the label, registration or use of glyphosate.

But UNSW medical Professor Bernard Stewart said the agency was the world's foremost authority on cancer research and typically evaluated a vast amount of research before including the most persuasive studies in its summary.

"Criticism from Monsanto, who has a clear financial vested interest, I'm unperturbed by criticism by someone such as them in the present context," Professor Stewart said.