The BBC's Margaret Gilmore: Contamination of other crops is inevitable

However, the government have refused an early parliamentary debate.

The report said that pollen and seed pollution by GM crops could not be avoided entirely and "acceptable levels" of contamination would have to be set, angering organic farmers.

Organic farmers must guarantee that their produce is entirely GM-free in order to be certified. The new report could force ministers to change the guidelines on the growing of GM crops to ensure that.

The government was asked for a debate on the report by its own backbenchers. Labour MP Joan Ruddock said there could be a "potential need to modify the Government guidelines on distances between such crops and organic food products, which clearly the public now very much support."

"Not new research"

Foods Minister Jeff Rooker: This is not new research

But Commons Leader Margaret Beckett said she could not promise an early debate on the issue. Food Minister, Jeff Rooker, told the BBC he would study the report but said: "This is a desk study - it's not new, original research."

The government-sponsored research was carried out by the John Innes Centre, one of Europe's leading research institutes on GM crops, and contains evidence that GM plants can cross-pollinate unmodified strains. The Ministry of Agriculture, Fisheries and Food commissioned the report.

Up to now, the government has denied organic growers' fears that their crops are at risk from pollen carried by wind and insects. The "buffer zones" currently required around GM oil seed rape or fodder maize are 200 metres wide. For sugar beet, the safe distance is 600 metres.

Hybrids possible

But earlier this year, research by the organic pressure group, the Soil Association, showed that more than 80% of rape seed pollen is carried by bees and that bees can fly at least three miles. Wind would carry the pollen further.

Patrick Holden, Soil Association: Government operating a "pollute now, worry later" policy

An earlier study by the National Pollen Research Unit also concluded there was a real threat of GM plant material contaminating organic farms many miles away.

It is understood the latest report includes evidence that one percent of organically-grown plants in any one field could become GM hybrids because of pollen spread. It concludes that contamination by seed or pollen cannot be entirely eliminated.

In a hole

Catherine Fookes, campaigns manager for the Soil Association, said: "The Government has dug itself into a real hole, because the results show you cannot prevent contamination with the current distances."

Friends of the Earth food campaigner, Adrian Bebb, agreed new rules are needed, saying: "The Government has got to go back to the drawing board and establish whether GM farming and non-GM farming can co-exist in this country. This latest report suggests they can't."

GM crops are now being grown on four farm-scale sites in England:

• Mortimer, near Reading, Berkshire
• Glentham, Lincolnshire
• Lyng, Norfolk
• Watlington, Oxfordshire.

Crops on a fifth large site were burned after the Soil Association threatened to withdraw the farms certification. The farmer's organic beans were growing just six metres away from a GM crop field.