The findings cast a cloud over the prospects of growing the modified crops in Britain, suggesting that farmers who try them out for one season will find fields blighted for a decade and a half.

Financed by GM companies and Margaret Beckett's Department of the Environment, Food and Rural Affairs, the report effectively torpedoes the Government's strategy for introducing GM oilseed rape to this country.

Ministers have stipulated that the crops should not be grown until rules are worked out to enable them to "co-exist" with conventional ones. But the research shows that this is effectively impossible.

The study, published by the Royal Society, examined five sites across England and Scotland where modified oilseed rape has been cultivated, and found significant amounts of GM plants growing even after the sites had been returned to ordinary crops. It concludes that the research reveals "a potentially serious problem associated with the temporal persistence of rape seeds in soil."

The researchers found that nine years after a single modified crop, an average of two GM rape plants would grow in every square metre of an affected field. After 15 years, this came down to one plant per square metre - still enough to break the EC limits on permissible GM contamination.

Last night Pete Riley, the director of GM Freeze, said; "It is becoming clearer and clearer that it is going to be impossible to grow GM crops in Britain."