Private labs in the US are said to have found significant residues of the glyphosate weed-killer, marketed by agrochemicals giant Monsanto as Roundup, in samples of honey, soy sauce, infant formula and even breast milk and urine.

Tests by the Abraxis lab found glyphosate residues in 41 of 69 honey samples and in 10 of 28 soy sauces, the news agency Reuters said, while tests by the Microbe laboratory detected the chemical in three of 18 breast milk samples and in six of 40 infant formula samples.

Although testing is seen to have increased over the past two years, Reuters quotes US lab operators as saying requests have skyrocketed since the end of March, when the French-based World Health Organization (WHO) research unit IARC announced plans to classify glyphosate as "probably carcinogenic to humans."

The report came to no specific conclusions about the dosage glyphosate requires to trigger cancer, however.

Monsanto, which is fighting the classification, has asked WHO to retract its report. In the meantime, three Chinese citizens have filed a lawsuit against their country's Ministry of Agriculture, seeking to force it to make public a toxicology report supporting Roundup 27 years ago.

 "According to physicians and other food safety experts, the mere presence of a chemical itself is not a human health hazard. It is the amount, or dose, that matters," Monsanto's senior toxicologist said in a public blog, adding that "trace amounts are not unsafe."

In the past, US regulators have repeatedly determined glyphosate to be safe, and a review of the chemical's safety by Germany for the EU last year established no link to cancer. However, critics say they fear that glyphosate is now so pervasive in the environment that extended exposure even to trace amounts can be harmful.

In light of the evidence turned up by the lab studies, they contend that glyphosate is more prevalent in the environment than previously believed.