Ultimately, the results weren't clear-cut and are expected to increase the rancorous debate over biotechnology roiling most of Europe.
The results of the $9.6 million, three-year study designed to gauge biotechnolgy's effects on wildlife left both sides of the genetic engineering debate claiming victory.
Fields of genetically engineered oilseed rape and beets were more harmful to wildlife than plots of conventionally grown crops. Biotech maize fields, though, were more hospitable than conventionally grown maize plots.
The three engineered crops were spliced with bacteria genes to make them more resistant than conventional crops to certain herbicides.
The study's conclusions said more about the impact herbicides had on wildlife than about biotechnology itself, the authors said.
The British government intends to use the results to help decide whether to allow commercial growing of engineered crops.
There are currently no commercial biotech crops grown in Britain or Europe, where polls consistently show a majority of people are opposed to genetically modified foods.
In the United States up to a third of some crops are genetically modified. The USDA approves all field trials of those crops, but labeling is not required when the food is sold in stores. Current, the FDA asks for companies using GM crops in food to voluntarily submit tests confirming product safety.
Biotechnology foes fear that genetically modified crops, often called GM crops, could harm the environment, including the possible emergence of new herbicide-resistant weeds that could cause havoc in the countryside. They also fear the new crops could be unhealthy.
"The results are clearly important to the debate about the possible commercialization of GM crops," said Les Firbank, the head of the research team. "But they also give us new insights that will help us conserve biodiversity within productive farming systems."
Environment Secretary Margaret Beckett said the government would examine the results of the study and the public debate expected to erupt over the findings.
The report said the mixed results arose because herbicide-resistant biotech crops gave farmers new options for weed and pest control.
During the study, scientists grew biotech crops and traditional crops side by side, using different weedkillers on each, and monitored the wildlife in the fields.
Insects such as bees in beet crops and butterflies in beet and spring rape were recorded more often in and around the conventional crops because there were more weeds to provide food and cover.
There were also more weed seeds in conventional beet and rape crops than in their biotech counterparts. The seeds are important to the diets of some animals, particularly some birds.
However, there were more weeds in and around the biotech maize crops, more butterflies and bees around at certain times of the year and more weed seeds.
Environmental groups criticized the narrow scope of the study and its use of the weedkiller Atrazine on the conventional maize crops. Atrazine is a highly toxic but effective chemical that the government has decided to phase out.
Critics said that the herbicides used on the modified maize were less toxic than Atrazine. Therefore, valid comparison between the two types of maize couldn't be made.
Results for a fourth crop in the trials, winter-sown oilseed rape, are expected to be published in 2004.