Monsanto Co, the world's largest seed company, and its brethren of global biotech crop developers are spreading the word that as far as the safety of their genetically modified grain goes, the science is solidly on their side.

The message of "settled" science has become the rallying cry for defenders of the crops and food commonly referred to as GMOs as they push back against consumers, environmentalists, lawmakers and others who want the crops labeled, restricted or banned.

"We believe the science is settled," Andrew LaVigne, CEO of the American Seed Trade Association, said in an interview.

But critics of the products say that is not the last word on the issue.

Some international scientists are challenging the assertion and say many scientific studies show concerns with crops whose DNA has been spliced in ways not seen in nature.

On Tuesday, a group with backing from institutions in , the United States and Europe said it would undertake the longest, largest and most definitive study of GMOs to date to try to settle the debate once and for all. 

The $25 million study of 6,000 rats to be fed a GMO corn diet is designed as an independent examination of the health impacts of GMO corn and the herbicide used on it. The research is to be done in  and western Europe over two to three years. ()

"The science on these GMOs is not settled by a long shot," said Bruce Blumberg, an endocrinology expert at the University of California, Irvine, who sits on the study review board. "Studies that were done by the manufacturers are the main ones showing safety, and those have an inherent conflict of interest." 

Monsanto, the leading developer of GMO crops, has stacks of research underscoring the safety of its products. Many U.S. university scientists also back the safety of GMOs, as does the U.S. government.

Karen Batra, a spokeswoman for the  Industry Organization, says "merely a handful of studies" point to health or safety issues, and all have been debunked.

Still, the debate rages on. Some biotech crop critics worry about pesticide residues in GMO foods, while others worry about what impact the crops have on the environment.

"The claim that there is a consensus among scientists that GM food products are safe ... is simply a PR campaign sponsored by the industry," said Dave Schubert, of the Salk Institute for Biological Studies in San Diego, California.