The World Trade Organization (WTO) has ruled that the European Union illegally stopped imports of genetically modified organisms (GMOs) from the US.
There have been many protests against GM crops
The case was instigated by the US, Canada and Argentina, which brought the complaint before the WTO in May 2003.
They said an EU moratorium on GM food crops, in place from 1998 to 2004, was about protectionism, not science.
The EU said it needed the block to allow it to gather data and find out how best to update GMO rules.
In a preliminary decision released on Tuesday, the WTO said that the EU-wide moratorium effectively amounted to a ban, breaking international trade rules.
The complainant nations say there is no scientific evidence for the EU action, which was an unfair barrier to producers or exporters of biotech foods wanting to do business in Europe.
The final ruling is due later this year.
Two years ago the moratorium was lifted and a modified strain of sweet corn, grown mainly in the US, was allowed onto the market.
But Washington continued with the WTO case because it wanted to be sure approvals for GMO sales were being decided on scientific rather than political grounds.
However the EU's executive arm, the European Commission, says there are now tough but fair laws in place, and that all GMO approvals will be processed on case-by-case scientific criteria.
"Our system aims to guarantee the unity of the internal market and complete safety for human health and the environment," a commission spokesman said.
There is a wariness in Europe about GM foods, in comparison to the US where they are more generally accepted.
Biotech crops, including corn and soybeans that have been genetically modified to resist insects or disease, have been widely grown in the US for years.