Genetically modified crops may not remain self-contained, according to a government study published on Monday.

The report also found that after GM oilseed rape was grown, it would take 16 years before conventional crops could be grown without fear of breaching the maximum one per cent contamination threshold.

Tough new rules are to be imposed on future trials of GM crops as a result.

Andrew George, spokesman for food and rural affairs, said: "The prime minister and the agriculture secretary are hell bent upon forcing GMs on the UK. But decisions should be based on sound science, rather than the make-do science that the government seems happy with.

"Consumers don't want it and supermarkets are not prepared to stock it. Farmers and growers would be mad to take on the liability when there is simply no commercial market for it.

"The decision on GM is likely to have irreversible consequences. The government must re-set a more rational timetable for measured consideration, instead of bowing to pressure from the United States biotech giants."

Forum Response: National Farmers Union

Martin Haworth, NFU director of policy, said: "The NFU, like everyone else, awaits the results of the field scale trials and the government's decision on whether commercial growing will be allowed.

"The NFU recognises that there may be considerable potential benefits to farmers, to the environment, and to the public in general from the introduction of biotechnological innovations into UK farming. But we also recognise that some people have serious concerns about the application of the technology.

"If the government does approve commercial growing of GM crops, the NFU believes this should be on a case-by-case basis following rigorous food safety and environmental impact assessment.

"But it is also fundamental that farmers should continue to have access to conventional and organic seed varieties and that systems are put in place to prevent conflict between farmers and help ensure freedom of choice for farmers and consumers."

Forum Response: Countryside Alliance

Simon Hart, chief executive of the Countryside Alliance, told ePolitix.com: "As always we want to see sound science on this issue.

"If there is even a slight health or environmental risk from growing GM crops on a commercial scale, then we should not tread this path, but we cannot condemn them without proper evidence and research.

"If science supports the growing of these crops on a commercial scale then we are in favour of them."