Monsanto's cash cow weedkiller Roundup probably causes cancer, according to a new report from the United Nation's cancer research organization.

The International Agency for Research on Cancer (IARC), published a report in the journal Lancet Oncology on Friday with findings on three pesticides, including Monsanto's best-selling brand.

The report found that the herbicide glyphosate — the active chemical ingredient in Roundup — and two insecticides, malathion and diazinon, were "probably carcinogenic." The conclusion was based on "limited evidence" of cancer among humans.

Researchers also found "convincing evidence" among animals for classifying the insecticides tetrachlorvinphos and parathion as "possibly carcinogenic."

The IARC, a branch of the World Health Organization, based in Lyon, France, says its classifications were made by an expert panel but are not binding.

"It remains the responsibility of individual governments and other international organizations to recommend regulations, legislation or public health intervention," the IARC said.

Monsanto was quick to point out that this classification contradicts earlier findings in the US and elsewhere.

“We don’t know how IARC could reach a conclusion that is such a dramatic departure from the conclusion reached by all regulatory agencies around the globe,” said Monsanto vice president for global regulatory affairs Philip Miller in a statement.

Monsanto invented glyphosate in 1974 and, marketed as Roundup, it became the world's best selling herbicide, representing about 10% of Monsanto's annual revenue. It's now also produced generically.

Roundup's dominance went hand-in-hand with the development of corn and soybeans genetically modified to resist the herbicide and Monsanto's Roundup Ready soybeans now account for a staggering 90% of the US crop.

"The general population is exposed (to glyphosate) primarily through residence near sprayed areas, home use and diet, and the level that has been observed is generally low," the IARC statement said.

In 1991, the US Environmental Protection Agency re-evaluated a previous classification of glyphosate as possibly cancer causing, and concluded there was "evidence of non-carcinogenicity" in humans.

Monsanto opposed the classification, saying in a statement that the IARC failed to produce any new data to support the classification, ignored studies that found the chemical safe and that the IARC report fails to establish a causal link between the chemical and an increased incidence of cancer.

"We urge anyone who wants to know more about glyphosate to look at the conclusions reached by regulatory authorities in developed countries that rigorously consider all available data, published and unpublished, in a comprehensive evaluation," the statement read.

“The IARC process is not a risk assessment," Prof Alan Boobis, Professor of Biochemical Pharmacology at Imperial College London told the UK-based Science Media Centre. "It determines the potential for a compound to cause cancer, but not the likelihood. Exposure assessment in epidemiological studies on the effects of pesticides is notoriously difficult. "

“These conclusions of IARC are important and should be taken into account when evaluating these pesticides, but that must also take into account how the pesticides are used in the real world. In my view this report is not a cause for undue alarm.”