The French government immediately asked the country's health watchdog to investigate the findings further and called on European authorities to "take all necessary measures to protect human health", including an "emergency suspension" of imports of the maize variety in Europe.
Frédéric Vincent, European Commission spokesman on health and consumer issues, said the European Food Safety Authority, EFSA, would examine it in detail.
“If it will be ascertained that the study indeed has scientific groudings, the Commission will draw the consequences,” he said.
French study results
Gilles-Eric Seralini, a biologist at the University of Caen, and colleagues said rats fed on a diet containing NK603 - a seed variety made tolerant to dousings of Monsanto's Roundup weedkiller - or given water with Roundup at levels permitted in the United States, died earlier than those on a standard diet.
The animals on the genetically modified (GM) diet suffered mammary tumours, as well as severe liver and kidney damage, according to the peer-reviewed study which was published in the journal Food and Chemical Toxicology and presented at a news conference in London.
The researchers said 50% of male and 70% of female rats died prematurely, compared with only 30% and 20% in the control group.
In a telephone conference call with US reporters, Seralini noted that GM animal studies typically conclude after three months, likely because companies behind genetically modified foods don’t want to know the long-term consequences of their products, the Washington Post reported.
“After four months” Seralini said about his own long-term study on 200 rats, “the tumours began”, the Post reported.
“After one year, there was a ... high increase in the number of tumours,” said Seralini, who is also the president of CRII-GEN’s scientific board. He said that most of the female rats had two or three tumours.
Some experts sceptical
Experts not involved in the study were sceptical, with one accusing the French scientists of going on a "statistical fishing trip" and others describing its methods as well below standard.
Monsanto spokesman Thomas Helscher said the company would review the study thoroughly. However, he added: "Numerous peer-reviewed scientific studies performed on biotech crops to date, including more than a hundred feeding studies, have continuously confirmed their safety, as reflected in the respective safety assessments by regulatory authorities around the world."
Genetically modified organisms are deeply unpopular in Europe but dominate major crops in the United States after Monsanto introduced a soybean genetically altered to tolerate Roundup in 1996.
Experts asked by reporters to review the scientific paper advised caution in drawing conclusions from it.
Tom Sanders, head of the nutritional sciences research division at King's College London, noted that Seralini's team had not provided any data on how much the rats were given to eat, or what their growth rates were.
"This strain of rat is very prone to mammary tumours particularly when food intake is not restricted," he said. "The statistical methods are unconventional ... and it would appear the authors have gone on a statistical fishing trip."
Mark Tester, a research professor at the Australian Centre for Plant Functional Genomics at the University of Adelaide, said the study's findings raised the question of why no previous studies have flagged up similar concerns.
David Spiegelhalter of the University of Cambridge said the methods, statistics and reporting of results were all below standard. He added that the study's untreated control arm comprised only 10 rats of each sex, most of which also got tumours.
While supporters of GM crops say previous studies have overwhelmingly pointed to their safety, critics argue there is still limited information about the long-term effects since the crops have only been around for just over 15 years.
In France, the government said it had asked its health and safety agency to assess the study and had also sent it to EFSA.
"Based on the conclusion ..., the government will ask the European authorities to take all necessary measures to protect human and animal health, measures that could go as far as an emergency suspension of imports of NK603 maize in the European Union," the French health, environment and farm ministries said in a joint statement.
Seralini, the scientist at the centre of the latest research, previously raised safety concerns based on a shorter rat study in 2009. His new study takes things a step further by tracking the animals throughout their two-year lifespan.