High-powered technological advances mean that new types of GM food are being developed almost daily. It is estimated that 60% of processed foods now contain such ingredients, for example soya.

As yet there is no evidence that these foods have an impact on human health.

But fears have been raised by research such as that controversially carried out by Dr Arpad Pusztai, who found that rats fed on GM potatoes suffered a weakened immune system and damage to vital organs. This includes an enlarged stomach wall and a shrunken brain.

Some scientists believe that tinkering with the natural genetic structure of food is bound to have a damaging impact on health, which might only become clear in the longer term.

Others, involved in the development of GM food, say what they do is no more dangerous than the cross fertilisation and selective breeding methods farmers have applied to their crops for centuries.

Unpredictable nature

Dr Mae-Wan Ho, of the Biology Department, Open University, warns that because no gene ever functions in isolation, there will almost always be unexpected and unintended "side-effects" from the gene or genes transferred into an organism.

She said many new genes that are inserted into food stuffs are taken from disease-causing virsues - genetic parasites that have the ability to invade cells and insert themselves into the cell's genetic make-up.

As such they have the potential to cause genetic damage and unpredictable physiological and biochemical effects.

There is also the potential that the exchange of genes could lead to the creation of brand new disease-causing organisms, made up of genetic material from many different species.

Dr Ho said: "We simply do not have sufficient understanding of the principles of physiological regulation to enable us to categorize those genetic modifications that will pose a risk and those that do not."

The unpredictability of genetic modifications has been illustrated by an experiment on yeast which was genetically engineered to produce hgher levels of enzymes to break down sugar.

Boosting the enzyme levels, however, led to the build-up of a toxic compound called methyl glyoxal.

Increased antibiotic resistance

Many scientists are concerned about the use of antibiotic-resistant "marker" genes in genetically engineered crops.

They fear that exposure to these genes will eventually lead to the development of antibiotic resistance in the bacteria found in the intestine of animals - or humans - who eat the crops.

In turn many micro-organisms may build up antibiotic resistance, and antibiotics may eventually lose their ability to combat disease.

Serious concerns have already been raised over a genetically modified maize produced by the Swiss firm Novartis.

The maize contains a gene which can generate resistance to antibiotics, and which the UK Government's Advisory Committee on Novel Foods and Processes has warned could rapidly neutralise medicine such as penicillin.

Allergic response

Genetically engineered food also has the potential to give rise to new allergens.

Scientists fear that many of these will not be detected before wholesale release into the food chain because no one can say what to test for.

A transgenic soybean containing a brazil-nut gene has already been found to be allergenic, as has a transgenic yeast engineered for increased rate of fermentation.

Dr Vyvyan Howard, a foetal and infant toxico-pathologist from the University of Liverpool, said genetically modified foods would have to be treated and tested like pharmaceutical drugs.

He said: "We are going to have to treat these plants like pharmaceutical agents.

"We consume food in a lifetime in tonnes, whereas with a drug you would expect to take it for a couple of days in milligrammes."

Dr Ian Taylor, scientific political adviser for the environmental charity Greenpeace, said his organisation had serious concerns about the potential health impact of genetically modified products.

He said: "There is a huge domain of scientific uncertainty about the impact of these things.

"The medium or long-term effects of introducing genetically engineered organisms into the environment or the food chain are unknown and unpredictable.

"No one is even able to ask the questions about what the impact will be of introducing genes never before part of the human diet."