The BBC programme The Big Dinner sent a food parcel picked at random to government food laboratories in Norwich which runs a commercial GM testing service. One in five of the samples tested contained GM soya.
Of the 20 samples The Big Dinner tested, 10 contained soya and of those 10, two - Bird's Eye Southern Fried Chicken Nuggets and Ross Bacon Burgers - contained genetically modified soya. Neither indicated this on the packaging.
Michael Antoniou, a clinical geneticist who has worked in the field for 20 years said: "I'm very concerned - we are all unwitting guinea pigs for the GM food experiment. In the medical field a new product would be tested on a controlled group before being made widely available. This doesn't happen with food."
The companies said that because soya and GM soya were mixed in the US it was impossible to tell whether the soya contained GM material or not.
A spokesman for Ross said yesterday: "There is no legal requirement to label genetically modified food at the moment. We in common with other food manufacturers have agreed a voluntary code of labelling.
"We are changing our products' labelling but it takes time. It is impossible because all soya coming in now contains a proportion of genetically modified material."
A spokeswoman for Bird's Eye said that the company was introducing voluntary labelling of soya protein during the course of 1998: "The entire food industry is making use of soya protein sourced from the United States 1997 harvest which because the crop is not segregated by definition means that some of the material may be derived from GM soya beans," she said.
"The new labelling is already featured on two of our products and the remaining products will carry the new GM labelling over the next few months according to our packaging renewal programme."