Dr Jean Emberlin, Director of the National Pollen Research Unit, believes the danger of contamination has been seriously underestimated.

She says the government should stop the controversial large-scale cultivation of GM crops, which is planned to start in a matter of weeks.

Concerns over genetically-modified food in the last month have put the government under pressure to halt development until further tests are carried out.

Pallab Ghosh: The study follows a government decision last summer

The basis for Dr Emberlin's warning is a study which says that pollen from maize can be dispersed over much greater distances than the 200m "exclusion zone" currently required around test sites.

Dr Emberlin's research was commissioned by the Soil Association, which promotes organic food and is opposed to genetic engineering.

It followed the government's refusal last summer to order the destruction of a GM maize crop bordering an organic farm in Devon.

The government's Advisory Committee on Releases to the Environment (Acre) maintained there was little or no risk of cross-pollination.

Craig Samms: "The risk of cross-pollenation is 400 times greater"

But Craig Samms, of the Soil Association, says the new report does indicate a real risk to farmers growing GM free crops as well as the consumers who believe they are buying organic produce.

"Nobody knows what the risk is to people because this technology has been introduced without any of the normal checks you would expect.

"Genetic engineering in medicine is subjected to strict containment rules," Mr Samms added, "But here you have a technology that is just being planted in the countryside with no real idea of what the implications are".

Bees 'ignored'

It said there was a one in 40,000 chance of pollen spreading the 2km to the organic farm.

But Dr Emberlin's analysis of data from numerous studies around the world suggests that the chance of bees carrying pollen had been ignored by Acre and that the risk was actually one in 90.

"The lack of acknowledgement of potential pollen spread concerns me," Dr Emberlin said.

"Once the pollen is out there it is very difficult to redress the situation. I don't think it would be wise to go ahead with large-scale planting of GM crops without knowing more about the possible repercussions."

Report 'biased'

The Department of Environment suggested that Dr Emberlin's report was biased.

It said international standards had been agreed although it admitted: "There are exceptions to every rule."

The department also rejected as "disingenuous" the suggestion that bees are a major factor in maize pollination in the UK.

"The issue of bees carrying maize pollen is a smokescreen to cast doubt on the competence and quality of Acre's advice," a statement said.

The Liberal Democrats and environment groups have welcomed the study.

Pete Riley, senior food campaigner at Friends of the Earth, said: "This new report highlights once again the appalling advice that the government has received on GM crops."

The report is published two weeks after a US biotechnology company, Monsanto, was fined £17,000 for breaking GM crop test site safety rules.

The firm had failed to maintain a six metre-wide barrier around a plot of genetically-modified oil seed rape in Lincolnshire.