John Gatehouse, a reader in biological sciences at Durham University, said the effects allegedly observed by Arpad Pusztai of the Rowett Research Institute almost certainly have nothing to do with genetic modification. Dr Pusztai's supporters said experiments in which he fed GM potatoes to rats showed the act of genetic modification, rather than toxins used, caused immune suppression and stunted growth.
Dr Gatehouse, one of three leaders of the government-funded project, said he had spoken out because of irresponsible press reports.
If any rats became ill it was probably because of a build-up of natural toxins in the potatoes as a result of the plants being grown from tissue cultures. "There might be a scientific explanation for what Dr Pusztai has observed but it is not necessarily to do with genetic modification."
An effect of culturing plants from tissue, which was how the potatoes in the experiment were grown, is that they regenerate with high levels of natural toxins, which can stunt the growth of laboratory animals. Potatoes are naturally rich in toxins but can be made more poisonous by growing the plants from tissue cultures. "It is an old effect and well known," Dr Gatehouse said.
Tony Blair stepped into the controversy with a claim that he would not hesitate to eat GM food. It echoed assurances about the safety of beef by the former Tory minister John Gummer, who tried to feed his daughter a beefburger.
The spokesman for the Prime Minister said he would not involve his children in the dispute, but it was clear the Government will not bow to pressure for a moratorium. Later, Downing Street said Mr Blair was "concerned" about calls for a ban on GM foods. "He is concerned there should be no headlong rush into something which is completely unnecessary, because there is absolutely no scientific evidence to suggest there is anything harmful about the food that is being produced at the moment," a spokesman said.
Sources said that any British attempt to ban US imports of the three licensed items already genetically modified - tomato paste, maize and soya - would spark a fresh trade war with America.
Other scientists have also criticised the interpretation of Dr Pusztai's results on grounds that natural toxins in raw potatoes can occur in surprisingly high amounts. Philip Dale, an expert on transgenic potatoes at the John Innes Centre in Norwich, said conventional cross-breeding experiments can throw up strains of potatoes that contain enough toxins to be poisonous to humans. "If we did the same sort of tests [as Dr Pusztai] on conventional lines of potatoes, we'd be throwing them out, but that does not condemn all potatoes."
More than 20 "independent" scientists - mostly from abroad - have signed a memo supporting Dr Pusztai, saying his work was of a sufficient standard to be published in a peer-reviewed journal, the only way new research is accepted by the scientific community at large.
But Dr Gatehouse said he had studied the unpublished "alternative report" written by Dr Pusztai - which he compiled as an answer to the criticism meted out in the Rowett official audit report into his work - and had found it contained serious problems.
"I think it looks to me to be too preliminary to publish. I personally would be very unhappy making those conclusions where the margins of error are at least as big as the effects being reported," Dr Gatehouse said.
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