The European Food Safety Authority declared on Tuesday that a banned strain of genetically modified maize poses no risk to health or the environment.
The move will once again pit the European Commission, which supports use of the maize, against the member states, most of whom do not.
The Monsanto MON810 maize "is as safe as its conventional counterpart with respect to potential effects on human and animal health", the independent risk assessor said after studying the strain.
In April, Germany became the sixth EU nation to ban the genetically-modified maize produced by the US biotech giant.
A month earlier EU nations refused to force Austria and Hungary to allow the cultivation of the GMO maize -- super resistant against insects -- defying a call from the European Commission.
France, Greece and Luxembourg have also banned cultivation of the GMO maize.
The case has upset Washington, which has warned Europe against using environmental issues as an excuse for protectionism amid disputes ranging from biotechnology to greenhouse gas emissions.
A European Commission spokeswoman said the EU executive would analyse the new findings and then make a recommendation to the 27 member states.
However environmentalists Greenpeace took no time in complaining that EFSA "has buried its head in the sand and ignored scientific evidence" on the negative effects of Monsanto's pesticide-producing GM maize on the environment. The European Commission's "blind reliance on EFSA's flawed opinions is likely to anger member states who feel scientific concerns on GM maize are not being addressed seriously", said Greenpeace EU GMO policy director Marco Contiero.
A four-year research project funded by the European Commission, the so-called Co-Extra report, declared early this month that genetically-engineered crops and conventional crops would have to be grown in segregated areas to meet environmental concerns about transgenic farming in Europe.
Given that fields in Europe are relatively small, and winds can spread pollen from transgenic crops over large distances, co-existence of novel and traditional crops will only be possible if they are grown in "dedicated zones", it said.
Genetically-modified crops have a gene, or genes, inserted into them in the lab so that they acquire traits that are useful to farmers.
They are widely grown in North America, South America and China.
But in Europe they have run into fierce resistance, led by green groups who say the crops carry risk through cross-pollination, potentially creating "super-weeds" that are impervious to herbicides.
Only a handful of genetically modified crop have been approved for cultivation in the European Union, but of them only the controversial MON810 maize strain is so far being grown, according to the European Commission.
It was approved back in 1998.
When EU environment ministers in March refused to force countries to lift their ban on MON810 only Britain, Estonia, Finland, the Netherlands and Sweden supported the EU executive's bid.
A source close to the European Commission told AFP at the time of the German ban that it might bring a revision of the European legislation on GM crops.