BRUSSELS, May 20 (Reuters) - The European Union executive said on Thursday it would freeze an approval procedure for a genetically modified maize developed by Pioneer Hi-Bred International (NYSE:PHB - news) following a U.S. study which found that other pest-resistant grains could kill butterflies.

"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 value or any bearing on that approval process," a European Commission spokesman told the EU's daily news briefing.

The spokesman for European environment commissioner Ritt Bjerregaard said the decision would affect the Pioneer product, which the Commission had been preparing to send up to EU ministers for approval after it failed last year to win a majority for clearance among lower-level experts.

"That will not be done until we have had the opportunity to evaluate this new information," the spokesman said.

Researchers in the United States reported in this week's Nature magazine that they had found leaves dusted with pollen from genetically modified ``Bt maize'' killed Monarch butterflies.

Bt-corn has genes from the bacterium Bacillus thuringiensis spliced into the plant genes which makes it resistant to a pest called the European corn borer.

The Commission spokesman said two varieties of Bt corn were already authorised for sale in the EU market "but we do not think there is any immediate danger."

He said the two products - made by life science groups Monsanto (NYSE:MTC - news) and Novartis - were not included in EU-wide seed catalogues and the executive believed they were planted in only a few fields.

The spokesman stressed the Commission believed it was much too early to draw any conclusions about whether the U.S. research had bearing on the already approved maizes.

"It's much too early to draw any kind of conclusion in any way, form or shape about products already on the market," the spokesman said, adding the Commission would not take any final decisions until the new research had been studied by its own scientists and government advisers.

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