Scientists are playing down fears following the discovery of a so-called “superweed” at a site where GM crop have been trialled.

The discovery of the charlock, a relative of oilseed rape, sparked worries when it remained unaffected after being treated with herbicide.

The creation of wild plants that pick up the traits of engineered crops is a threat often cited by anti-GM groups.

However, researchers claimed their work shows the chance of any such transfer is slim. They pointed to the study, which discovered the “superweed” as evidence, with it reinforcing the view that the environmental impact of GM crops was negligible.

Les Firbank, who led a consortium of scientists on the recent UK Farm-Scale Evaluations (FSEs) of genetically modified plants, said: “Herbicide-tolerant weeds tend to under-perform compared with wild type, so unless all its competitors have been sprayed out with the same herbicide, it won't thrive.

"There's lots of evidence for that," he told the BBC News website.

The study was conducted by Centre for Ecology and Hydrology (CEH) researchers and was looking for any evidence that genetic traits would pass to near-relatives growing wild in the field or nearby.

The team collected more than 95,000 seeds of wild relatives in and around the FSE trial sites and grew them up in greenhouses. These plants were then sprayed with Liberty (a glufosinate ammonium) to see if they had acquired herbicide tolerance - through their parents being pollinated by the GM rape.

The study revealed just two plants, brassica rapa or turnip rape, were showing resistance to treatment.

However, the plants are a very close relative to farmed oilseed rape and scientist said the discovery of some gene-flow is not a big surprise.

The team also toured fields, and found just one weed, a charlock, which showed no reaction to an application of Liberty.

DNA analysis on a leaf sample confirmed the gene trait from the engineered oilseed rape was present, but when the researchers returned the following year to the same field they could find no herbicide tolerance in seedlings of the charlocks growing there.

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