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The dispute over genetically modified crops will intensify today with news of the evolution of "superweeds", which are resistant to the powerful weedkillers that GM crops were engineered to tolerate.</p>The development, which comes as the sacked former environment minister Michael Meacher puts himself at the head of the anti-GM campaign, will be seized on by opponents of the technology as undermining its rationale.</p>It means that bigger quantities of weedkillers - not less, as the biotechnology companies have claimed - will be needed in GM-crop fields, adding to the already intensive agriculture that has wiped out much of Britain's farmland wildlife in the past four decades. Monsanto, the GM market leader, confirmed to The Independent</i> at the weekend that its solution for dealing with resistant weeds was to apply different weedkillers in new ways.</p>In yesterday's Independent on Sunday</i>, Mr Meacher accused Tony Blair, a GM supporter, of seeking to bury health warnings about GM produce by "rushing to desired conclusions which cannot be scientifically supported".</p>The revelations about superweeds have been communicated to the Government by an American academic specialising in weed control, who has posted a paper on the website of the official GM science review, led by Professor David King, the Government's chief scientific adviser. This will report soon in advance of a long-delayed decision, due this autumn, on whether GM crops should be commercialised in Britain.</p>The paper, by Professor Bob Hartzler of the Department of Agronomy at Iowa State University, reveals that in the past seven years, up to five weed species have been found with resistance to the herbicide glyphosate, best known by the Monsanto trade name Roundup. The resistance has come about not through gene transfer from GM herbicide-tolerant crops, as some have feared, but through natural evolution.</p>Glyphosate is a "broad spectrum" herbicide, meaning that, originally, it killed everything, including crops. GM crops were developed to be tolerant of the herbicide, so it could be applied throughout the growing season.</p>Two GM crops proposed for commercial growth in Britain, fodder beet and sugar beet, are glyphosate-tolerant. But weeds have been found in Australia, Chile, Malaysia and California and other areas of the US, that glyphosate cannot kill.</p>Greg Elmore, Monsanto's US technical manager for soybeans, said Monsanto was taking seriously the question of glyphosate resistance, tackling it with "weed control management practices".</p>With soybeans, he said, resistant weeds were controlled with a pre-planting "burn-down" (which kills everything), using 2,4-D, another weedkiller.</p>At least three of the resistant weeds had evolved where glyphosate was being used with non-GM crops, he said, adding that it was far from the only weedkiller for which weeds had evolved resistance - as many as 70 weeds were resistant to some weedkillers.</p>Pete Riley, Friends of the Earth's GM campaigner, said: "Companies like Monsanto have spun GM crops and their weedkillers as having less impact on the environment, but the fact of resistant weeds undoubtedly means more weedkillers, and means the impact on the environment will be greater.</p>"These discoveries remove a central plank from the whole argument for GM crops."</p>Yesterday, Mr Meacher listed a series of reports and findings suggesting that the full impact of GM technology was still dangerously unpredictable. Many of the health tests carried out were "scientifically vacuous", he said. </p>