The Food And Drink Federation, which represents food manufacturers, is launching a web site devoted to the issue and is distributing CD-Roms on the subject to schools.

The main concerns about modified food are that its long-term effects on health have not been exhaustively tested and that altered crops could damage the environment, by harming insects that feed on them. Cross pollination between plants could lead to "super weeds" that can resist herbicides, making them impossible to kill.

Geneticists are raising the alarm because within the next few weeks soya harvested last autumn in America will reach the shops - the beans can be found in processed foods, including pizza, margarine, chocolate and ice cream.

The concern is that 15% of last year's American crop - five times more than in 1996 - was genetically modified to enable the beans to resist a herbicide used to keep weeds down. The figure is expected to double during this year's harvest.

The American farmers have not had to segregate modified and unmodified crops and so it is now increasingly likely that manufacturers could be unintentionally adding modified soya to food.

A voluntary labelling agreement has just come in to force which labels all food containing soya protein as being genetically modified.

Critics say the system does not empower consumers to make informed purchases because nearly all products will be labelled as containing modified soya, regardless of whether they really are.

The Genetics Forum, which is opposed to the rapid introduction of modified food, says the labels are meaningless as they do not have the force of law behind them. The modified soya looks and tastes like the original product, and there is no law to force shops to admit that an item may contain genetically altered ingredients.

The forum is also critical of the voluntary code that applies to products containing soya protein but not soya oil, which is used in many products, such as flour and chocolate.

"What we've got now is a mishmash that we're not happy with," says Susan Casey from the forum. "The system we have is being put together on an ad hoc voluntary basis. We think if the population was aware of how modified food is creeping into shops without them knowing they would be horrified. You only have to look at the demand for organic produce to realise the importance people place on knowing what is in their food and where it has come from."

There is no proven health risk associated with modified food, but opponents claim this does not prove it is safe because there have been no long-term tests. Malcolm Walker, chairman of Iceland, the supermarket chain, is one of the industry's strongest opponents of, what he terms, "Frankenstein food".

"It takes seven years or so to get a new pill on the market, but these new foods are getting approval at the drop of a hat," he says. "The Americans are purposefully not separating the modified and unmodified soya because they know, if given the choice, we would go for the unmodified. There can be no categorical assurance that these new foods are not bad for us, the long-term tests have not been done. And soya is the thin end of the wedge, later this year we will probably be seeing altered maize and wheat."

Iceland is due to announce that its own-brand products are free of genetically altered ingredients. It buys from suppliers who have secured unmodified ingredients.

The Food and Drink Federation can be found at www.foodfuture.org.uk