"(We have) been saying for years that Bt plants are not an ecological solution to pest control," said Gill Lacroix of Friends of the Earth Europe's biotechnology programme.

"The European Union should take immediate action to stop commercial planting of GM maize in Europe now. There is no benefit of such crops either for consumers or the environment, but rather a very significant risk," Lacroix said in a statement.

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 Cornell team, whose research was published in the science journal Nature, fed monarchs milkweed plants dusted with pollen from Bt-corn. The butterflies ate less than those fed on normal milkweed and nearly half the larvae died.

Although the research was limited to laboratory tests and there was no evidence of what effect the transformed pollen has on monarch butterflies in the wild, the study highlighted fears about the effects of genetically-modified organisms (GMOs) on the environment.

Lacroix said biodiversity in Europe was already declining and "the introduction of genetically engineered crops can only make that already dire situation even worse."