Large fragments of genetically modified DNA could be entering the human food chain, according to findings of a key Government committee obtained by The Observer.

An independent study by the Advisory Committee on Animal Feeding Stuffs reveals that alien genes used by scientists to modify crops are surviving the manufacturing process which turns GM crops into animal food.

The report heightens fears that products such as chicken, turkey, beef and pork may be contaminated with modified genes if the animals they are from were raised on GM feed. Until now Ministers and industry bodies have reassured consumers that a heating process kills any DNA in animal fodder.

Deputy Agriculture Minister Joyce Quin told the House of Commons last December: 'Ministry-funded work has confirmed that DNA is degraded during the processing of most animal feed.'

But the new disclosure that genetic material remains in the food chain will raise widespread concern. More than two million tonnes of GM crops are imported into the UK for animal feed.

The major concern will be that the DNA can transfer to bacteria in the gastro-intestinal tracts of animals fed on this material.

All animals, including humans, have bacteria in their gut which helps to fight disease as well as easing digestion. But some GM animal feed contains an alien gene which confers resistance to antibiotics. If this gene were to transfer to the bacteria in cows, it could make it harder for farmers to treat infections. Another concern is that farmworkers inhaling the dust from GM feed could be affected.

Environmentalists further fear that humans eating meat or dairy products from livestock fed on GM crops could be at risk, although so far there is no evidence that bacteria in human guts have been affected.

However, earlier this year, The Observer revealed the work of a German scientist who had found that genes from GM crops could be found in bacteria in the guts of bees.

Fears of this kind led the Government's independent advisory committee - which reports to the Food Standards Agency - to commission the consultants ADAS to monitor the manufacturing of animal feed.

The draft minutes of the committee, seen by this newspaper, state: 'The results indicate that DNA fragments large enough to contain potentially functional genes survived processing in many of the samples studied.'

Members of the committee had 'expressed surprise that so much DNA survived processing', the minutes add.

Greenpeace has called on the Government to take immediate steps to stop the continuing import of GM crops from the US for use in animal feed.

A spokesman for the environmental group, Andy Tait, said: 'It is beyond belief that even in the wake of the BSE crisis, independent research into the potential for GM to cross into the guts of animals was not done before these crops were allowed to market.'

The GM firm Monsanto has always said the genes used in its soya crops are 'inactivated' by heat processing during animal feed manufacture.

Biotech companies argue that Britons have been happily eating and drinking milk, chicken, pork and beef from livestock raised on GM-rich diets since the early Nineties and shown no ill effects.

A Food Standards Agency spokeswoman said the findings had been a surprise but the body did not believe the study raised food safety issues. 'While we now know that DNA does survive the manufacturing process in some samples there is no scientific proof that this then crosses into the guts of the animals that eat it.'

The agency would go on pressing for the compulsory labelling of food from animals raised on GM feed, she added.

The possibility of the suspect DNA entering human food was first raised in the US in the early Nineties.