Genetically modified rice has been spreading illegally for years in China, officials have admitted, triggering a debate on a sensitive aspect of the food security plan in the world's most populous nation.
Two strains of GM rice were approved for open-field experiments but not commercial sale in 2009. In January, the agriculture ministry said "no genetically modified cereals are being grown in China" outside the test sites.
But in April, an environment ministry official told the weekly Nanfang Zhoumo that a joint investigation by four government departments had found that "illegal GM seeds are present in several provinces because of weak management".
The agriculture ministry did not respond to an AFP request for clarification.
According to the website for the European Union's Rapid Alert System for Food and Feed, European countries found foodstuffs from China containing GM rice 115 times between 2006 and May this year.
The campaign group Greenpeace says GM rice seeds have been in China since 2005, and were found at markets in Hubei, Hunan and Jiangxi provinces last year, Fang Lifeng, a Chinese agriculture specialist with the group, told AFP.
Beijing is pro-biotechnology and has already allowed several GM crops to be grown, including cotton, peppers, tomatoes and papayas, and has authorised imports of GM soy and corn for the food industry.
But rice -- the key staple in the diet of the country's more than 1.3 billion people -- is a much more sensitive question.
"Two-thirds of Chinese eat rice every day," said Tong Pingya, a highly respected agronomist who blasted Chinese scientists for "treating the people like guinea pigs" at a conference in May chaired by Vice-Premier Li Keqiang.
"China does not need this genetically modified rice, as it produces enough and even exports a bit," Tong told AFP.
When the National People's Congress, China's rubber-stamp parliament, met last year, around 100 researchers wrote to deputies asking them to revoke authorisations for the use of experimental GM grains, including a strain of corn as well as the two rice types.
They also demanded a public debate and clear labelling of products containing genetically modified organisms.
Backers of GM rice argue that it is more drought-resistant, offers better yield, and -- in the case of the variety containing the Bt gene -- allows pesticide use to be dramatically cut.
"It should be possible to authorise commercialisation around 2012-2013, but the state will probably not allow them to be used on a wide scale" in the near future, said Ma Wenfeng, a grain market analyst with the consultancy CNagri, which has links to the agriculture ministry.
According to Ma, the new varieties represent "an advance in biotechnology" and will ultimately be accepted.
For their part, environmentalists and some Chinese scientists warn against the as-yet unknown long-term consequences of using GM rice for biodiversity and human health.
Whether using them is in farmers' interests is an open question, according to Greenpeace's Fang, because "GM seeds cost two to five times more than ordinary seeds" and "in terms of yield, there isn't really a difference".
GM rice strains developed in Chinese laboratories also raise questions about intellectual property.
The Bt gene is patented by the US agribusiness giant Monsanto, which could demand royalties and compensation from China if that variety is commercialised.