The UK Government said it would consider the latest research on GM crops carefully before deciding whether to allow the plants to be commercialised.
"I have said consistently that the government is neither pro nor anti-GM crops - our overriding concern is to protect human heath and the environment, and to ensure genuine consumer choice," Environment Secretary Margaret Beckett said.
Birds like the skylark have suffered under modern farming methods
She spoke after scientists revealed the results of a three-year study of genetically modified herbicide-tolerant crops.
The tests of three biotech crops found the cultivation of two - an oilseed rape and a beet crop - to be more harmful to many groups of wildlife than their conventional equivalents.
The production of a third biotech plant, maize, was shown to be kinder to other plants and animals than the normal crop.
The slightly contradictory results of the £6m study allowed all sides in the debate on novel crops to claim "victory".
GeneWatch, a science and policy group which looks at the issues surrounding genetic engineering, described the results of the trials as "shocking".
"The UK's farmland wildlife has been decimated by intensive farming," said director Sue Mayer. "If we grow herbicide tolerant crops here it may be the final nail in the coffin for some species."
And environmental campaign group Friends of the Earth said the trials had a dark warning for the British countryside.
"The impact of GM crops on wildlife is very dramatic," said Tony Juniper, the group's director. "The government has got no alternative but to stand by its pledge to ban GM crops in the UK."
He claimed failure to act could lead to birds such as the skylark becoming extinct in Britain.
The pro-biotech campaign group CropGen interpreted the results differently.
It said the so-called farm-scale evaluations (FSEs) showed GM technology, if managed properly, could benefit the environment as well as farmers and consumers.
"Today is a momentous one for UK agriculture," CropGen said in a statement. "GM Maize is good for farmers, better for biodiversity and is ready for commercial cultivation."
Many commentators recognised the FSEs represented only another stage in the GM debate and that much more research was required to prove the effectiveness of GMs - and that work would have to be done on a crop-by-crop basis.
Sir Ben Gill, president of the National Farmers' Union, said: "The decision on whether to allow these crops to be grown commercially now lies with the government and it must be taken on a case-by-case basis using all the available evidence, including these trials.
"It must be remembered that these results primarily look at farmland biodiversity. Studies to develop best practice for the management of these crops, should they be approved, will be important if farmers are to deliver maximum environmental benefits."
Scepticism and caution
Dr Guy Poppy, an ecologist at Southampton University, said it would be wrong to make "all or nothing decisions" about GM technology based on the FSE results.
"What we need to do now is ensure that the benefits of GM outweigh any risks, as is the case for maize.
"It is imperative that we don't throw the baby out with the bathwater and lose the benefits that some GM crops can offer."
The trial results will now be assessed by Acre (Advisory Committee on Releases to the Environment), the agency that will advise the government on their implications.
A decision by ministers on whether to commercialise the crops could come later this year, or early in 2004.
Shadow environment secretary David Lidington said: "These results deserve careful study. I shall want to hear the views of other scientists on the findings published by their professional peers.
"The fact that the impact of different GM seeds on wild plants and invertebrates varies so markedly shows that we must proceed with scepticism and caution."