Seeds of some genetically modified crops can endure in soil for at least 10 years, scientists have discovered.
Researchers in Sweden examined a field planted with experimental oilseed rape a decade ago, and found transgenic specimens were still growing there.
This was despite intensive efforts in the intervening years to remove seeds.
No GM crop has been found to endure so long; and critics say it shows that genetically modified organisms cannot be contained once released.
Tina D'Hertefeldt from Lund University led the team of scientists that scoured the small field which had hosted the GM trial 10 years ago looking for "volunteers" - plants that have sprung up spontaneously from seed in the soil.
"We were surprised, very surprised," she told BBC News. "We knew that volunteers had been detected earlier, but we thought they'd all have gone by now."
Eradication effort
Presenting their findings in the journal Biology Letters, the researchers note that after the trial of herbicide-resistant GM rape, the Swedish Board of Agriculture sprayed the field intensively with chemicals that should have killed all the remaining plants.
And for two years, inspectors looked specifically for volunteer plants and killed them.
We should assume that GM organisms cannot be confined, and ask instead what will become of them when they escape
Professor Mark Westoby
This is much more effort than would usually be deployed on a normal farmer's field.
But even so, 15 plants had sprung up 10 years later carrying the genes that scientists had originally inserted into their experimental rape variety to make them resistant to the herbicide glufosinate.
Non-GM varieties were used in the 10-year-old study as well, and some of these had also survived.
"I wouldn't say that the transgenic varieties are able to survive better," said Dr D'Hertefeldt. "It's just that oilseed rape is a tough plant."
Jeremy Sweet, a former head of the UK's National Institute of Agricultural Botany and now an independent consultant on biotech crops, agreed.
"It's been known for some time that oilseed rape is a bit of a problem because of the survival of its seed," he told BBC News.
"It means that if farmers want to swap [from growing GM rape] to conventional varieties, they will have to wait for a number of years."
Growth industry
Rapeseed - often known by its Canadian name canola - is the fourth most commonly grown GM crop in the world, after soya beans, maize and cotton.
An industry organisation, the International Service for the Acquisition of Agri-biotech Applications (ISAAA), calculated recently that more than one million square kilometres of land across the world are now dedicated to growing GM plants.
Europe accounts for only about 0.1% of that total, with a single maize variety the single transgenic food plant being grown.
Many European countries, including the UK, have yet to implement legislation on the thorny issue of how fields of genetically modified crops could co-exist with others that farmers are keen to keep free of transgenic material.
The Lund research does not deal with the flow of genes into neighbouring fields, or whether transgenes can transfer into wild plants growing nearby.
But Tina D'Hertefeldt believes legislators do need to take note of her findings.
"What we are saying is they also need to take into account the temporal aspect," she said.
Professor Mark Westoby, a plant ecologist from Macquarie University in Australia, had a more blunt assessment.
"This study confirms that GM crops are difficult to confine," he said.
"We should assume that GM organisms cannot be confined, and ask instead what will become of them when they escape."
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