The International Agency for Research on Cancer's (IARC) rules on assessing substances for carcinogenicity say it can consider only published research.
New data from a large American study, had not been published when it produced its report concluding glyphosate was "probably carcinogenic" in 2015.
A legal case against Monsanto, taking place in California, involves 184 individual plaintiffs who cite the IARC assessment and claim exposure to RoundUp gave them cancer. They allege Monsanto failed to warn consumers of the risks. Monsanto denies the allegations.
Monsanto said the data on glyphosate could and should have been published in time to be considered by IARC, and that the failure to publish it undermined IARC's classification of glyphosate.
American Chemistry Council (ACC) president and chief executive Cal Dooley said: "These allegations suggest that an IARC Monograph has become nothing more than a rubber stamp for predetermined outcomes.
"We encourage all countries and organisations that support the Monographs programme to join us in calling for an investigation into whether IARC officials knowingly withheld data that proved a lack of association between glyphosate and cancer, and other monographs should be evaluated to determine whether similar manipulation has taken place.
"Today’s revelations lend even greater urgency to the need for fundamental reform of IARC’s Monographs program, and because IARC’s glyphosate monograph is fatally flawed and no longer credible, it should be immediately withdrawn."