The Telegraph's science editor, Roger Highfield, reports on the strange disappearance of what was hailed as the world’s worst leakage of genetically modified crops into regular strains.

Four years ago, researchers reported finding cobs of genetically modified maize in Oaxaca, Mexico, suggesting that GM maize (corn) from the US had invaded a traditional maize variety.

This contamination was loudly trumpeted by anti-GM campaigners, even though the GM variety had no adverse effects on people. What happened then was strange. The magazine Nature disowned the original paper by researchers at the University of California in Berkeley.

The paper had sparked a protest to Nature by 100 biologists and was disowned by the Mexican government after its scientists could not repeat the experiment. The anti-GM lobby portrayed the row as an attempt to discredit the research and as part of a biotech industry vendetta.

The story becomes stranger. A two-year study by Ohio State University researchers, published in the Proceedings of the National Academy of Sciences, finds there has been no spread of GM strains into native crops.

The researchers gathered more than 153,000 seeds from 870 maize plants in 125 fields in Oaxaca, for the first survey of foreign "transgenes" in native varieties, and found no evidence of contamination. The finding surprised the researchers, said Prof Snow, because millions of tons of GM grain were imported from the US each year for processed food and animal feed.

It all makes the findings of the original Berkeley study seem extraordinary. What was claimed as the worst incident of GM contamination has disappeared into thin air. The refutation might not get quite the coverage the original scare generated, however.