Test results expected to lead to the commercial production of genetically modified maize in the UK are invalid, says the former minister who set them up.
Protesters have uprooted GM crops
An EU ban on a weedkiller called atrazine means the three-year tests must start again, claims former environment minister Michael Meacher.
The results, due to be published on Thursday, are expected to suggest weedkillers used on the GM crop are less damaging to the environment than those used on conventional maize varieties.
A government spokesman denied the ban meant the tests were flawed.
She said herbicides other than atrazine, which is suspected of causing cancer, had also been used in the trials.
Mr Meacher said: "We need to try the trials again with a different herbicide to see what the comparison is between that and the GM one.
"I cannot see that the government could logically, consistently, or morally go ahead when the comparison is exposed to everybody as not being a valid or a real one."
More research was also needed on the health effects of GM food and
"cross-contamination" between GM crops and other plants, he added.
The findings of the trials are likely to have a huge influence on the final decision on commercial production of GM crops in the UK.
It was reported that this week's results would show herbicides used with two of the three GM crops tested - oilseed rape and sugar beet - were more damaging to insects and plants than normal weedkillers.
Friends of the Earth's GM campaigner Pete Riley said: "The banning of atrazine makes the GM maize trials practically worthless.
"The government has spent millions of pounds of public money on trials comparing one dodgy system with another, instead of serious research to find a sustainable way to grow maize.
"The indications are that tests on GM oil seed rape and beet will show they have more impact on wildlife than their non-GM equivalents.
"These trials were never enough to give GM crops the green light, but they may provide enough information to give them the red one."
A Department for the Environment, Food and Rural Affairs
(Defra) spokeswoman said the results were not flawed as the trials had tested other weedkillers as well as atrazine.
She added that Britain would not make any decision on GM crops without agreement within the EU.