Most of our milk, cheese, yoghurt and pork comes from animals raised on genetically-modified food, a campaigning group claims.

The Soil Association said the widespread use of GM crops, such as maize and soya, flies in the face of consumer opposition to the controversial technology.

It believes the public is being kept in the dark about farmers and supermarkets' reliance on GM animal feed because there is no legal requirement to mention it on food labelling.

A spokesman said: 'This GM stealth invasion of the UK food chain is denying consumers their right to make fully- informed choices.'

It is thought more than 70 per cent of dairy cattle and pigs are given GM feed at some point.

Campaigners said there is evidence that genetically-modified DNA is getting into the milk and meat that reaches consumers.

A Daily Mail campaign over the socalled 'Frankenstein foods', coupled with mass consumer opposition, was instrumental in the decision by all the major supermarkets to ban GM ingredients from their own-brand foods.

But these bans, which were imposed more than five years ago, did not extend to the feed given to animals. Only Marks & Spencer took a stand by removing GM ingredients from the diet of meat and dairy animals. GM feed is also banned by organic farmers.

Waitrose, the Co-op and Sainsbury's offer a few non-organic meat and dairy items produced from non-GM feed. These include Sainsbury's 'farm promise' milk. Most modified animal feed crops have been altered to include a gene that gives them resistance to spraying with certain chemicals, which means they can survive being treated with weedkillers.

A spokesman for the Soil Association, which has long lobbied against GM, said: 'Most consumers are unwittingly eating food produced from GM crops every day.'

Unlike the dairy and pig industry, poultry farmers have largely dropped the use of GM feed. However, a third of eggs come from GMfed hens.

A spokesman for the British Retail Consortium, which speaks for supermarkets, said: 'Our view is that all the evidence shows that there is no transference of GM material from feedstuffs to meat and dairy products.

'Given that this is the case, there is nothing to be gained from labelling the use of GM feed.'

The Agriculture Biotechnology Council, which represents GM companies such as Monsanto and Bayer Cropscience, accused the Soil Association of 'scaremongering'.

'If they have any evidence to support their claims, it should be submitted to the official regulatory authorities,' a spokesman said.