The EU, however, does not have a method of detecting whether imported feed contains Bt10.

Such a measure would affect firms other than just Syngenta, but the full scope of the EU's requirement is not yet clear.

On Tuesday EU Health and Consumer Protection Commissioner Markos Kyprianou briefed the full Commission on the issue and proposed "a measure be taken to ensure that the unauthorised GMO Bt10 cannot enter the European Union," Commission spokesman Philip Tod told a briefing.

Kyprianou received the backing of the Commission.

"If all goes well, it (the measure) could be adopted by the weekend," Tod said.

"What we are talking about is that...imports from the United States would have to be accompanied by an analytical report from an accredited laboratory certifying that they are free of Bt-10," he said.

Tod said Kyprianou's measure was expected to go to an EU standing committee on Friday.

The Commission, the EU's executive arm, has asked Syngenta to provide a detection method for Bt-10, which got mixed up with an authorised biotech maize called Bt-11. Syngenta has not yet done so.

The mix-up occurred between 2001 and 2004.

Up to eight kilos (17.6 lb) of Bt-10 seeds, within a 100-kg lot of Bt-11 seed, arrived in France for research between 2001 and 2004 from US suppliers, not for commercial growing. Two kg went to Spain. All the seeds have since been destroyed.

Some 1,000 tonnes of Bt-10 maize also entered the EU as food and animal feed but it is not clear to which countries. Around 70 percent of this is thought to be animal feed. EU authorities still do not know how, when and where all this happened.

The Commission has previously said it might consider imposing a temporary ban on GMO animal feed made from maize gluten originating in the United States.