Dr Jean Emberlin, Director of the National Pollen Research Unit, has produced evidence to show that pollen from maize can be dispersed over much greater distances than has been accepted by government scientists.

At present, a 200-metre "exclusion zone" is set up around a GM maize crop undergoing trials and is considered a sufficient barrier to prevent cross-contamination of ordinary maize crops or sweet corn. But, Dr Emberlin, whose research was commissioned by the Soil Association, says bees or strong winds will take the pollen much further.

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

Environment Minister Michael Meacher told BBC Radio that he accepted there could be a contamination risk of around 1% at 200m under moderate speed wind conditions. But he said the 200-barrier should be sufficient to ensure the purity of nearby crops.

Michael Meacher: "I do take any new scientific data very seriously"

It was based on many years of research and recognised as adequate by the European Commission, the Origination for Economic Co-operation and Development, and the US authorities.

"If it is necessary and right to revise the criteria by which we assess the likelihood of cross-pollination, we will do so," he said.

Once again, he stressed the government's line on GM crops: "We are not going to allow commercial planting of GM crops until we are sure that we have the evidence to guarantee that there will be no damage to the environment - or indeed to human health."

Political pressure

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 Soil Association, which promotes organic food and is opposed to genetic engineering, asked Dr Emberlin to undertake this latest research after 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.

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.

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

"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".

'Disingenuous'

"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."

A statement from the Department of Environment released before Mr Meacher's radio interview 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," the statement read.

Liberal Democrat food spokesman Paul Tyler and environment spokesman Norman Baker welcomed the report.

"This report gives the lie to the dismissive attitude of both Conservative and Labour ministers. It is now clear that the risk is far greater than they have told us," they said in a joint statement.

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."