The shipment was tested in the United States and the positive results for Bt-10 were sent to Ireland to allow Dublin to stop the cargo on arrival, the EU executive said.

"The Irish authorities are taking necessary measures to ensure that the contaminated consignment does not enter the food chain," Commission spokesman Philip Tod told a news conference.

Last month the European Union blocked imports of maize from the United States unless shipments carried proof that they were free of Bt-10, which is not authorised for use either in Europe or the United States.

The curb will be reviewed at the end of October but the EU's food safety chief said last month the conditional ban may be extended if more contaminated products were discovered.

Syngenta said the impounding of the maize shipment in Ireland showed that the testing system for Bt-10 was working.

"The testing and certification implemented by the European Union is doing exactly what it is supposed to do," said a Syngenta spokesman.

USexporters send 3.5 million tonnes of corn gluten feed to Europe each year, a trade worth some 350 million euros ($440 million).

In March Syngenta said some of its maize seeds sent to the EU from the United States were mistakenly mixed with Bt-10. This insect-resistant strain is similar to Bt-11, a different GMO strain that is approved for distribution in the EU.

The maize mix-up occurred between 2001 and 2004.