English Nature this week expressed "deep concern" that large-scale farming in the United Kingdom of genetically modified crops resistant to herbicide could create weeds that cannot be killed.
The official conservation body said its fears are based on growing evidence from the United States that herbicide-resistant weeds are beginning to emerge.
Glyphosate is the most popular herbicide in the US. An English Nature spokeswoman said: "During the first couple of years glyphosate is extremely efficient and farmers using it have become accustomed to virtually weed-free fields. But nature is resilient and weeds are inevitably starting to show signs of tolerance."
Farmers are applying higher doses of glyphosate more often to their fields.
They may even need to use other chemicals to deal with the toughest weeds.
"The same patterns would almost certainly emerge in Britain," said English Nature. The issue "further brings into question industry claims that GM crops will make agriculture more sustainable," it added.
English Nature wants more research to measure the costs and benefits of new farm technologies. It said the government's trials of herbicide-tolerant oilseed rape and maize would not examine the long-term, cumulative impact.
But it was important the trials continue: "While they may not provide us with an answer to whether all GM crops are consistent with sustainable farming, their results will help fill in a vital part of the GM jigsaw."
Guy Poppy, a researcher at the Institute of Arable Crops Research, said: "Even if a weed becomes resistant, it will become resistant to one of a number of herbicides and you can move onto another. But, as with all chemicals, resistance can become a problem and needs to be managed."
Dr Poppy this week published a paper that suggests certain GM crops armed with pest-killing toxins could be less dangerous to some non-target species than conventional chemical insecticides. The study argues that interrelations between pests and non-pests must be taken into account when assessing GM crop impact.