After a two-year study identified a “shockingly” increased risk of cancer in rats exposed to the world’s biggest-selling weedkiller, and a GM maize crop resistant to it, The Sustainable Food Trust has called for a change in the regulatory approval process.

In the first-ever study to examine the long-term effects of Monsanto’s Roundup weedkiller and NK603 Roundup-resistant GM maize, scientists found that rats exposed to even the smallest amounts developed mammary tumours and severe liver and kidney damage.

The study revealed a fundamental weakness in the current regulatory approval process, which dates back to the 1940s, said Patrick Holden, founder of The Sustainable Food Trust.

“The research exposes a critical deficiency. The short duration of the required feeding trials fails to identify the long-term consequences of consuming these crops,” he added.

Mustafa Djamgoz, professor of cancer biology at Imperial College agreed. “A firm takeaway message for the regulatory bodies is that pesticides and crops have got to be evaluated and tested over longer periods,” he said.

The GM maize was rubber-stamped by EFSA in 2009 under the current 90-day approval process. However, rats live for 700 days, and the two-year study found that the first tumours were only diagnosed after four months.

Up to half the male rats and 70% of females died prematurely, compared with 30% and 20% in the control group. Across both sexes, rats that were fed Roundup in their water, or NK603 maize, developed two to three times more large tumours than the control group.

By the beginning of the 24th month, 50% to 80% of females in all treated groups had developed large tumours, with up to three per animal.

The study, published in Food and Chemical Toxicology, was described by Dr Michael Antoniou, molecular biologist at King’s College London, as “the most thorough research ever published into the health effects of GM food crops and the herbicide Roundup on rats”.

Monsanto promised to thoroughly review the study. EFSA said it would consider the paper,

However, some academics and pro-lobby groups criticised it. “The methods, stats and reporting of results are well below the standard I would expect,” said Prof David Spiegelhalter, a professor of risk at the University of Cambridge.

“This strain of rat is prone to mammary tumours,” added Prof Tom Sanders, head of nutritional sciences research at KCL.