Britons have unwittingly been eating banned GM rice imported from the United States for months, if not years, food safety experts fear.
Imports of the rice were stopped by the European Commission (EC) on Thursday. But investigations in the US show that it has long been "wide-spread" in grain destined to be shipped overseas.
It was first discovered in January that the banned crop, which has never received safety clearance, was contaminating export stocks of long-grain rice. But it was not until nine days ago that the US government informed importing countries.
European governments are furious that the Bush administration delayed warning them. And the row threatens ministers' plans for growing GM crops in Britain.
The unauthorised rice, codenamed LLRICE601, was developed by Bayer CropScience to tolerate weedkiller. It was tested on US farms between 1998 and 2001, but the company decided not to market it and never submitted it for official approval.
In January, it was found to have contaminated rice from Arkansas-based Riceland, the world's largest miller and marketer, which is responsible for one-third of the entire US crop.
In May, Riceland tested samples from "several storage locations", finding the contamination in a "significant" number. It concluded, in an official statement, that it was "geographically dispersed and random" throughout its rice-growing area.
Bayer officially notified the US government on 31 July. But it was a further 18 days before the Bush administration told importers, informing EU countries such as Britain just an hour before holding a press conference to make details of the contamination public.
On Thursday, the EC prohibited any shipments from the US unless they could be proved to be free of the banned rice. But it remains concerned that Britons and other Europeans may have been eating it for months, possibly years.
Britain has imported more than 42,000 tons of long-grain rice from the US since January, when the problem was first discovered. No one knows how much of this was contaminated, but the Food Standards Agency is planning to carry out tests on rice that has yet to be sold to the public.
The Arkansas government suspects that the crisis began when pollen from the rice tested on US farms spread to contaminate conventional crops. This would mean that it has been present - and presumably been exported - at least since 2001, when the trials stopped.
Richard Bell, the state's agriculture secretary, admits that the contamination is "widespread" and predicts it will show up again in this year's crop when it is harvested.
The Bush administration says that "there are no human health, food safety or environmental concerns associated with this rice". But the EC's Health and Consumer Protection Commissioner, Markos Kyprianou, says it must not be allowed to enter the food chain.
Bayer, which had no part in exporting the contaminated rice, says it is "co-operating closely" with the US authorities. But it says that while the matter is being investigated, it cannot say when it first knew of the problem .