The row over the environmental impact of genetically modified crops has flared up among North American scientists as new research suggests commercially grown transgenic maize poses no threat to the monarch butterfly.

Six studies to be published in the Proceedings of the National Academy of Sciences seek to answer fears raised two-and-a-half years ago.

Two research teams were concerned that pollen from Bt maize, which carries a toxin-producing bacterial gene to protect the crop against pests, would kill monarch butterfly caterpillars.

The new work concludes that pollen from the main varieties of Bt maize has a negligible effect on the larvae.

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The principal authors of the original papers contributed data to the new work and praised its reliability. But last week the same scientists sent a qualifying memo to the US Environmental Protection Agency warning that the research may have ignored an important element of the toxic risk.

John Obrycki, professor of entomology at Iowa State University, and one of the three scientists who contacted the EPA, said the new research's remit was too narrow.

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Professor Obrycki is concerned that monarch caterpillars living on milkweed plants close to maize fields would encounter and eat not only Bt pollen but fragments of the anthers, a part of the plant that contains far higher levels of the toxin.

Mark Sears, professor of environmental biology at Guelph University, Canada, dismissed this as "unsupported by data". He said: "The anti-GMs need to get their act together and speak with documented evidence rather than innuendo."

His own risk assessment calculates that, at most, 0.007 per cent of the US monarch butterfly population - between 500 and 2,000 insects - could annually consume sufficient levels of Bt pollen to suffer an adverse effect.

Other studies show that one variety of Bt corn did kill significant numbers of caterpillars but this is being phased out for unrelated commercial reasons.

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