A study also found that after growing GM oilseed rape, it could take a farmer up to 16 years to grow a conventional rape crop that would comply with the maximum 1 per cent GM contamination threshold.

The findings were released by the Department for the Environment, Food and Rural Affairs just three days before the results of the official farm trials and as about 1,000 anti-GM campaigners protested against the technology in Central London. Opponents of GM crops immediately hailed the findings as proof that the controversial process would damage the environment and dent farm profits.

Bees could carry GM pollen distances of 16 miles, the researchers found. It had been thought that bees carried pollen to a limit of just over two miles. And unless weeds from GM crops were strictly controlled, conventional crops risked contamination for up to 16 years, they concluded.

A Whitehall source admitted that the results might put extra pressure on senior ministers not to rush into widespread commercial planting of GM crops. The source suggested that it would increase caution inside the Government and that the most likely outcome was for decisions on GM crops to be made on individual cases.

Elliot Morley, the Environment Minister, signalled a harder line to companies when he insisted that in any future trial the purity of seeds would be closely inspected.

Ministers were embarrassed by an incident when Bayer CropScience released an unlicensed variety of oilseed rape on 22 farms during crop trials last year. The firm is not to face prosecution over the incident in England, it was announced yesterday, although a decision has yet to be made by the Procurator Fiscal for the unlawful planting that took place on three Scottish farms. The Government, however, has ordered that in any future trials companies must pay for the extra monitoring and inspection of their seed varieties.

Mr Morley said: “We are determined to have effective systems in place to ensure consumer choice whatever the future of GM in this country. It is a global product and we must have systems of control to ensure seed purity and consumer choice.”

In a further blow to the biotechnology industry, Margaret Wallstgrom, the European Union’s Environment Commissioner, who is in London for talks with Margaret Beckett, the Environment Secretary, accused the companies of “trying to lie” and “to force” unsuitable crops on European farmland. She blamed American-style lobbying tactics for fuelling public suspicion and fears about the new technology.

She said: “I hope they have definitely learnt a lesson from it, and especially when they now try to argue that this will try to answer the problems of starvation in the world.

“After all, why didn’t they start with such products, so they could prove to the world that this was exactly what they were interested in doing? It will solve starvation among shareholders, but not the developing world, unfortunately.”

The pro-GM lobby said that the findings should not be viewed “in terms of black and white”. A spokesman for the industry group The Supply Chain Initiative for the Modification of Arable Crops said that the risk of pollen travelling long distances was very small and the research showed it was possible for GM and non-GM crops to be grown in the same farming regions without any detrimental effects.

He said: “The whole case of co-existence between crops in agriculture revolves around the concept of thresholds. Farming takes place in the open air and practical thresholds are achievable.”

Bob Fiddaman, a spokesman for the National Farmers’ Union, who tested GM oilseed rape on his 1,100-acre farm in Hertfordshire, was also relaxed about the findings. “After the first crop, farmers would always control weeds,” he said.

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