(Reuters) - U.S. consumer groups, scientists and food companies are testing substances ranging from breakfast cereal to breast milk for residues of the world's most widely used herbicide on rising concerns over its possible links to disease.

The focus is on glyphosate, the active ingredient in Roundup. Testing has increased in the last two years, but scientists say requests spiked after a World Health Organization research unit said last month it was classifying glyphosate as "probably carcinogenic to humans."
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Tests by Abraxis found glyphosate residues in 41 of 69 honey samples and in 10 of 28 soy sauces; Microbe tests detected glyphosate in three of 18 breast milk samples and in six of 40 infant formula samples.

North Dakota State University agronomist Joel Ransom reported to the U.S. Wheat Quality Council in February that tests he ordered showed traces of glyphosate in several U.S. and Canadian flour samples.