Although the lead researcher's past record as a critic of the industry made other experts wary of drawing hasty conclusions, the finding will stoke controversy about the safety of GM crops.

Monsanto was not immediately available for comment but the group has in the past repeatedly said its products are safe and there is no credible evidence of any health risk to humans or animals from consuming GM crops.

It is expected to create particular waves in France, where fierce opposition to genetically modified organisms (GMOs) led to a ban on growing such plants.

In an unusual move, the French-led research group did not allow reporters to seek outside comment on their paper before its publication in the peer-reviewed journal Food and Chemical Toxicology and presentation at a news conference in London.

Gilles-Eric Seralini of the University of Caen and colleagues said rats fed on a diet containing NK603 - a seed variety made tolerant to dousings of Roundup - or given water containing Roundup at levels permitted in the United States died earlier than those on a standard diet.

The animals on the GM diet suffered mammary tumours, as well as severe liver and kidney damage.

The researchers said 50 percent of males and 70 percent of females died prematurely, compared with only 30 percent and 20 percent in the control group.

HARD TO EVALUATE

Alan Boobis, a professor of biochemical pharmacology at Imperial College London who was not involved in the research but sent an emailed comment on it just after its publication, said the study had been designed in a way that made it difficult to evaluate the significance of its findings.

"For example, there does not appear to be a statistical analysis of the mammary tumours," he said, adding that "these occur quite often in untreated animals."

Supporters of GM crops say previous studies have overwhelmingly pointed to their safety, but critics argue there is still limited information about the long-term effects since the crops have only been around for just over 15 years.

Seralini was part of a team that has flagged previous safety concerns based on a shorter rat study in a scientific paper published in December 2009, but this new study takes things a step further by tracking the animals throughout their two-year lifespan.

Monsanto said at the time of the earlier research that the French researchers had reached "unsubstantiated conclusions."

Seralini believes his latest lifetime rat tests give a more realistic and authoritative view of risks than the 90-day feeding trials that form the basis of GM crop approvals, since three months is only the equivalent of early adulthood in rats.

France's Jose Bove, vice-chairman of the European Parliament's commission for agriculture and known as a fierce opponent of GM, called for an immediate suspension of all EU cultivation and import authorisations of GM crops.

"This study finally shows we are right and that it is urgent to quickly review all GMO evaluation processes," he said in a statement.

"National and European food security agencies must carry out new studies financed by public funding to guarantee healthy food for European consumers."