One of the world’s most popular weed killers – and the most widely used kind in the US – can “probably” cause cancer, according to United Nations health chiefs.

The World Health Organisation’s (WHO) cancer arm has announced that best-selling ‘Roundup’, produced by Monsanto, contains an active ingredient that is “classified as probably carcinogenic to humans”.

Amateur gardeners and professional farmers have been urged to “think very carefully” about using the popular herbicide after a report was published in clinical journal Lancet Oncology on Friday.

The report revealed glyphosate was “classified as probably carcinogenic to humans”.

It also said there was “limited evidence” that the key ingredient was carcinogenic in humans for non-Hodgkin lymphoma.

Monsanto, the world’s largest seed company, said scientific data does not support the conclusions and called on WHO to hold an urgent meeting to explain the findings.

The report was also posted on the website of the International Agency for Research on Cancer (IARC), the France-based arm of the WHO.

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

Concerns about glyphosate on food have been a hot topic of debate in the United States recently, and contributed to the passage in Vermont last year of the country’s first mandatory labelling law for foods that are genetically modified.

In 2013, Monsanto requested and received approval from the US Environmental Protection Agency for increased tolerance levels for glyphosate.

The US government says the herbicide is considered safe.

But Andreas Kortenkamp, professor of human toxicology at Brunel University, London, said that the people who are most at risk are those applying the weedkiller to their plants.

He said: “Professional gardeners would use industrial strength glyphosate to totally wipe their garden of all plants. Amateur gardeners can also buy it as Roundup in a formulation which is not as strong.

“Anyone who sprays it could get a whiff of it. People should be very careful with this stuff and consider whether they need it. Home gardeners should hand weed to be on the safe side.”

Glyphosate is mainly used on crops such as corn and soybeans that are genetically modified to survive it and the recent report said the weed killer has been detected in food, water and air after it has been sprayed.

However, it also said glyphosate use is generally low in and near homes where the general public would face the greatest risk of exposure.

The evidence for the organisation’s conclusion was from studies of exposure, mostly agricultural, in the United States, Canada, and Sweden that were published since 2001.

Carcinogens are substances that can lead to cancer under certain levels of exposure.