"We would of course want to apply the precautionary principle and there's no way any new products can be approved where this information might have...any bearing on that approval process," said Peter Jorgensen, spokesman for acting EU Environment Commissioner Ritt Bjerregaard.

"But if there are any problems related to this, then of course appropriate action...could also have a bearing on products already approved," he added.

The latest hold-up in approving new transgenic crops threatens to sour transatlantic trade relations even further at a time when the EU and the United States are locked in conflict over Europe's ban on U.S. beef produced using hormones.

The EU is facing intense pressure from Washington to speed up the approvals process. The U.S. says its farmers lost around $200 million in maize exports to Europe last year because genetically modified (GM) crops already grown in the United States are not approved for use in Europe.

American biotech firms complain that European distrust of the new technology is also costing them millions of dollars.

"We continue to be concerned that science takes a back seat to politics again and again on issues of food safety," a U.S. official in Brussels said. "That just erodes consumer confidence even further." Researchers from Cornell University in the United States reported in this week's Nature magazine they had found leaves dusted with pollen from genetically modified "Bt maize" killed Monarch butterflies.

Jorgensen said the Commission would wait for further studies before deciding what to do next and, in the meantime, would not ask EU environment ministers, due to meet next month, to take a final decision on whether to approve the Pioneer maize.

Pioneer, which is in the process of being bought by Dupont Co, accused the Commission of inconsistency.

"They don't see any immediate danger from the products already approved, but are casting doubts about ours, which is very similar to the Monsanto maize," said Brussels-based Pioneer spokesman Thierry Habotte.

"We fully support further studies, but they must be scientifically-based. My impression is that the agenda of some politicians goes rather further than the science," he added.

A spokesman for Monsanto said the study should be taken in context and stressed that the research in no way replicated real conditions in the field.

"One study can't be treated as proof," said Monsanto European spokesman Tom McDermott. "We need to consider how the findings compare with other evidence and how the Bt technology compares with other means of controlling the same pest."

Novartis cultivated 22,000 hectares of its new maize last year, the company said. Austria and Luxembourg still have national bans in place on planting the Novartis maize.

Earlier, in the light of the U.S. study, environmental group Friends of the Earth called for a total ban on GM crops.