Premature deaths and sickness were concentrated especially among females.

At the 14-month stage of experiment, no animals in the control groups showed any signs of cancer, but among females in the “treated” groups, tumours affected between 10 and 30 percent of the rodents.

“By the beginning of the 24th month, 50-80 percent of female animals had developed tumours in all treated groups, with up to three tumours per animal, whereas only 30 percent of controls were affected,” it said

Males which fell sick suffered liver damage, developed kidney and skin tumours and digestive problems.

Breaking with a long tradition in scientific journalism, the authors allowed a selected group of reporters to have access to the paper, provided they signed confidentiality agreements that prevented them from consulting other experts about the research before publication.

Asked to respond, the French unit of Monsanto said “it is too soon to make a serious comment because we have to evaluate the study.

As soon as it is available, our experts will look closely at it to give their scientific assessment.”Green groups say GM crops could be dangerous to health and the environment, although this claim has so far found no traction in large-scale studies.

The Monsanto spokesman said that “more than 300 peer-reviewed studies” had found that GM food was safe.

In 2009, the European Food Safety Agency (EFSA) panel on GM organisms determined that NK603 was “as safe as conventional maize”.

“Maize NK603 and derived products are unlikely to have any adverse effect on human and animal health in the context of the intended uses,” it said, delivering a judgement based in part on a 90-day feeding study on rats.