Over recent weeks UK poultry and egg suppliers have been telling retailers that it is increasingly difficult for them to guarantee that the feed they use is entirely GM free.

Tim J Smith, Tesco group technical director, said that soya is the best source of protein for livestock and as soya producers are increasingly turning to GM soya, it means they are producing less non-GM soya. Therefore, there is a shortage of non-GM feed, causing a supply issue on a global scale.

Mr Smith said that over 80 per cent of the world's soya is now modified and that farmers worldwide are opting for modified soya maily because modified crops are more resistant to certain pests and diseases.

Mr Smith added that because so much soya is modified and because of the way crops are planted, processed and transported, it is likely that non-GM soya crops contain low levels of GM soya.

The new DNA testing regime that Tesco has put in place has identified that the risk of finding GM material in non-GM feed is increasing.

"We could not continue with a promise we cannot be sure it is possible to keep and we want to be upfront about the changes we are making," Mr Smith said.

He stressed that just because the feed ingredient in which poultry eat is genetically modified, it does not mean that the poultry and eggs Tesco sells will be genetically modified in any way.

The Food Standards Agency (FSA) has made it clear that genetic modification affects only the crop used in the feed, and the DNA from GM soya is not present in the meat of animals, nor in animal products such as eggs or milk.

The FSA is also clear that there is no risk to health from eating meat from animals that have been fed GM feed. In fact, meat products from animals fed on GM crops have been a standard part of many supermarket ranges for some time.

Mr Smith said: "We want to be clear with customers about food and that’s why we’re removing the GM-free guarantee for poultry and eggs. We are not the first UK supermarket to reach the conclusion that a non-GM policy on poultry feed is unsustainable, and we won’t be the last. Asda and Morrisons already allow GM feed for poultry products, and our suppliers also work with other UK retailers."

The poultry industry body in the UK, the British Poultry Council has commented on the news, telling ThePoultrySite: "GM crops are highly regulated in terms of health and environmental safety. They have been used in the UK to feed livestock, including poultry, destined for the retail supply chain around the world for the last 15 years.

"The availability of non-GM soya has reduced significantly in recent years as growers produce more GM varieties. The likelihood of accidental GM presence is much greater than ever before and so it is no longer possible to guarantee that feed is entirely GM-free."