Publication of the results of the UK's study of genetically modified crops, the biggest conducted anywhere in the world, has already sparked controversy.
GM and non-GM maize was compared
Scientists who tested three GM crops found more damage to wildlife from two, oilseed rape and sugar beet, than from their conventional equivalents.
A third biotech plant, a maize, was better for wildlife than normal crops.
But some of the researchers say part of the trials will need repeating, as a key herbicide used is being phased out.
The dispute over the validity of the results bears out the concerns of some opponents of GM crops, who have been warning that the trials were flawed.
The farm-scale evaluations (FSEs) looked at how GM herbicide-tolerant (GMHT) crops might affect farmland weeds and insects, both of them important food sources for wild creatures.
The GMHT crops can be sprayed with a particular weedkiller and still prosper while "pest" plants in the field are killed.
Critics said the trials were too narrowly focused, ignoring other possible effects like damage to consumers' health, cross-pollination with other plants, harm to soil organisms, and the long-term transfer of modified genes.
They also said the maize tests were invalid and would have to be repeated, because the herbicide used on the conventional maize was atrazine, to be phased out by the European Union.
They were concerned because they thought its eventual replacement might well be more wildlife-friendly, cancelling out the advantage which GM maize appears to enjoy.
These doubts clearly weighed with the researchers. In a commentary on the FSEs, they write: "The actual effects... were remarkably consistent for each crop.
"This finding gives us confidence that the findings would represent what would actually happen under large-scale growing, unless the management regimes altered somewhat, for example if... atrazine was no longer allowed on maize crops..."
The chairman of the FSE independent scientific steering committee, Professor Christopher Pollock, sees no problem with the maize tests.
He told BBC News Online: "Atrazine is in current use, so the data are consistent and have value. Calls for the trials to be restarted are perhaps less than wholly appropriate."
The leader of the research team, Dr Les Firbank, was less certain. He said: "If the management systems changed, we'd have to recalibrate our results, look again at our data, and possibly get new data. But we wouldn't have to start again from scratch."
But another member of the team, Professor Geoff Squire, went further. He told BBC News Online: "The difference we found between conventional and GM crops is explained by the timing. The conventional herbicides zap the weeds fairly early on, leaving them time for a later surge.
Different crops require different management practices
"With GM herbicides you can get into the crop and get the weeds later, so they have no chance to grow back.
"With atrazine, it kills so much of the wildlife because of its persistence and its toxicity.
"Obviously, if atrazine is withdrawn, we'll have to look at maize again. This is a package, the GM crop and the herbicide.
"If either element changes, we shall have to revisit it. I think this is a view shared among the research team."
The scientists grew the GM plants and their conventional equivalents side by side, and then observed the wildlife among the crops and at the field margins.
Some insects, such as bees (in beet crops) and butterflies (in beet and rape), were found more frequently 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 GM counterparts. But some groups of soil insects were found in greater numbers in the GM beet and rape crops.
In contrast, growing GM maize was better for many groups of wildlife than conventional maize. There were more weeds in and around the biotech maize crops, more butterflies and bees at certain times of year, and more weed seeds.
The researchers stress that the differences they found are not a result of the way in which the crops have been genetically modified.
They arose because the GM crops gave farmers taking part in the trial new options for weed control. That is, they used different herbicides and applied them differently.
The results will now be assessed by Acre (the Advisory Committee on Releases to the Environment), which will advise the government on their implications.
Ministers will decide whether to allow GM crops to be grown commercially in the UK later this year, or early in 2004.
The GM rape tested was a spring variety. A winter rape is also being investigated and the results of its evaluation will be published next year.