But environmentalists said a verdict in the politically charged case, brought against the European Union by the United States, Argentina and Canada, could take even longer after trade judges agreed to hear scientific opinion.
WTO judges had initially been expected to issue a ruling in September or October, but officials said that it had been put back until the end of March to give both sides more time to make their case and to let the judges question scientists.
The judges' decision to take evidence from scientists was seen as a victory for the EU, which had pressed for their views to be heard, while the United States and its allies had argued that this was unnecessary.
"I think that next spring is very optimistic (for a decision)," said Adrian Beeb of green activists Friends of the Earth.
In a previous trade row between the United States and the European Union involving food and health - that time over beef hormones - it had taken the WTO two years to gather scientific opinion, he noted. The EU eventually lost the case.
In bringing the case in August last year, the United States and its allies argued that the then 15-nation EU had flouted trade norms by not allowing any GMO crops to be grown or imported since 1998, in what amounted to a de facto ban.
The United States says that there is no scientific evidence for human health or environmental problems related to biotech products - two of the grounds on which WTO rules allow countries to bar imports.
Washington and its allies also argued the WTO had no need to hear scientists because the argument was over whether or not the EU had applied its own rules for approving GMO applications.
"The issue is that the EU has a mechanism and that it has not been applying it. Science does not come into it," said one trade diplomat from a country involved in the case.