Modified genes from crops in a GM crop trial have transferred into local wild plants, creating a form of herbicide-resistant "superweed", the Guardian can reveal.

The cross-fertilisation between GM oilseed rape, a brassica, and a distantly related plant, charlock, had been discounted as virtually impossible by scientists with the environment department. It was found during a follow up to the government's three-year trials of GM crops which ended two years ago.

The new form of charlock was growing among many others in a field which had been used to grow GM rape. When scientists treated it with lethal herbicide it showed no ill-effects.

Unlike the results of the original trials, which were the subject of large-scale press briefings from scientists, the discovery of hybrid plants that could cause a serious problem to farmers has not been announced.

The scientists also collected seeds from other weeds in the oilseed rape field and grew them in the laboratory. They found that two - both wild turnips - were herbicide resistant.

The five scientists from the Centre for Ecology and Hydrology, the government research station at Winfrith in Dorset, placed their findings on the department's website last week.

A reviewer of the paper has appended to its front page: "The frequency of such an event [the cross-fertilisation of charlock] in the field is likely to be very low, as highlighted by the fact it has never been detected in numerous previous assessments."

However, he adds: "This unusual occurrence merits further study in order to adequately assess any potential risk of gene transfer."

Brian Johnson, an ecological geneticist and member of the government's specialist scientific group which assessed the farm trials, has no doubt of the significance. "You only need one event in several million. As soon as it has taken place the new plant has a huge selective advantage. That plant will multiply rapidly."

Dr Johnson, who is head of the biotechnology advisory unit and head of the land management technologies group at English Nature, the government nature advisers, said: "Unlike the researchers I am not surprised by this. If you apply herbicide to plants which is lethal, eventually a resistant survivor will turn up."

The glufosinate-ammonium herbicide used in this case put "huge selective pressure likely to cause rapid evolution of resistance".

To assess the potential of herbicide-resistant weeds as a danger to crops, a French researcher placed a single triazine-resistant weed, known as fat hen, in maize fields where atrazine was being used to control weeds. After four years the plants had multiplied to an average of 103,000 plants, Dr Johnson said.

What is not clear in the English case is whether the charlock was fertile. Scientists collected eight seeds from the plant but they failed to germinate them and concluded the plant was "not viable".

But Dr Johnson points out that the plant was very large and produced many flowers.

He said: "There is every reason to suppose that the GM trait could be in the plant's pollen and thus be carried to other charlock in the neighbourhood, spreading the GM genes in that way. This is after all how the cross-fertilisation between the rape and charlock must have occurred in the first place."

Since charlock seeds can remain in the soil for 20 to 30 years before they germinate, once GM plants have produced seeds it would be almost impossible to eliminate them.

Although the government has never conceded that gene transfer was a problem, it was fear of this that led the French and Greek governments to seek to ban GM rape.

Emily Diamond, a Friends of the Earth GM researcher, said: "I was shocked when I saw this paper. This is what we were reassured could not happen - and yet now it has happened the finding has been hidden away. This is exactly what the French and Greeks were afraid of when they opposed the introduction of GM rape."

The findings will now have to be assessed by the government's Advisory Committee on Releases to the Environment (Acre). The question is whether it is safe to release GM crops into the UK environment when there are wild relatives that might become superweeds and pose a serious threat to farm productivity. This has already occurred in Canada.

The discovery that herbicide-resistant genes have transferred to farm weeds from GM crops is the second blow to the hopes of bio-tech companies to introduce their crops into Britain. Following farm scale trials there was already scientific evidence that herbicide-tolerant oilseed rape and GM sugar beet were bad for biodiversity because the herbicide used to kill the weeds around the crops wiped out more wildlife than with conventionally grown crops. Now this new research, a follow-up on the original trials, shows that a second undesirable potential result is a race of superweeds.

The findings mirror the Canadian experience with GM crops, which has seen farmers and the environment plagued with severe problems.

Farmers the world over are always troubled by what they call "volunteers" - crop plants which grow from seeds spilled from the previous harvest, of which oilseed rape is probably the greatest offender, Anyone familiar with the British countryside, or even the verges of motorways, will recognise thousands of oilseed rape plants growing uninvited amid crops of wheat or barley, and in great swaths by the roadside where the "small greasy ballbearings" of seeds have spilled from lorries.

Farmers in Canada soon found that these volunteers were resistant to at least one herbicide, and became impossible to kill with two or three applications of different weedkillers after a succession of various GM crops were grown.

The new plants were dubbed superweeds because they proved resistant to three herbicides while the crops they were growing among had been genetically engineered to be resistant to only one.

To stop their farm crops being overwhelmed with superweeds, farmers had to resort to using older, much stronger varieties of "dirty" herbicide long since outlawed as seriously damaging to biodiversity.

Q&A: What the discovery means for UK farmers

What's the GM situation in the UK?

No GM crops are currently grown commercially in the UK. Companies who wish to introduce them face a series of licensing hurdles in Britain and Europe and interest has waned in recent years amid public opposition.

Other firms have dropped applications in the wake of the government field scale trials that showed growing two GM varieties - oilseed rape and sugar beet - was bad for biodiversity.

The EU has approved several GM varieties and the UK government insists that applications will be considered on a case-by-case basis.

Where are GM crops grown?

Extensively in the wide open spaces of the US, Canada and Argentina. In Europe, Portugal, France and Germany have all dabbled with GM insect-resistant maize. Spain plants about 100,000 hectares (250,000 acres) of it each year for animal feed.

What is a superweed?

Many GM crop varieties are given genes that allow them to resist a specific herbicide, which farmers can then apply to kill the weeds while allowing the GM crop to thrive.

Environmental campaigners have long feared that if pollen from the GM crop fertilised a related weed, it could transfer the resistance and create a superweed. This "gene transfer" is what appears to have happened at the field scale trial site. It raises the prospect of farmers who grow some GM crops being forced to use stronger herbicides on their fields to deal with the upstart weeds.

Is it a big problem?

Not yet. Farmers in the UK do not grow GM crops commercially. If they did, then the scale of possible superweed contamination depends on two things: whether the hybrid superweed can reproduce (many hybrids are sterile) and, if it could, how well its offspring could compete with other plants. Herbicide-resistant weeds could potentially grow very well in agricultural fields where the relevant herbicide is applied. Most experts say superweeds would be unlikely to sweep across the UK countryside as, without the herbicide being used to kill their competitors, their GM status offers no advantage.

Some GM crops, such as maize, have no wild relatives in the UK, making gene transfer and the creation of a superweed from them impossible.

Is it a surprise?

On one level no, gene flow and hybridisation are as old as plants themselves. Short of creating sterile male plants, it's simply impossible to stop crops releasing pollen to fertilise related neighbours. But government scientists had thought that GM oilseed rape and charlock were too distantly related for it to occur.

The dangers of hybridisation where it does happen are well documented - experts from the Dorset centre behind the latest research published a high-profile paper in 2003 in the US journal Science showing widespread gene flow from non-GM oilseed rape to wild flowers.

Have superweeds surfaced elsewhere?

Farmers in Canada and Argentina growing GM soya beans have large problems with herbicide-resistant weeds, though these have arisen through natural selection and not gene flow through hybridisation. Experiments in Germany have shown sugar beets genetically modified to resist one herbicide accidentally acquired the genes to resist another - so called "gene stacking", which has also been observed in oilseed rape grown in Canada.

· David Adam