Rats fed a lifetime diet of Monsanto's genetically modified corn or exposed to its top-selling weedkiller Roundup suffered tumours and multiple organ damage, according to a French study published on Wednesday.

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

In an unusual move, the 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 per cent of males and 70 per cent of females died prematurely, compared with only 30 per cent and 20 per cent in the control group.

Seralini was part of a team that flagged previous safety concerns based on a shorter rat study in a scientific paper published in December 2009 but this 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. – Reuters