Sir, Joanna Blythman has rightly challenged supermarkets to resist moves by the British Poultry Council to drop their policy of only using non-GM feed ('We can't allow GM chicken feed', May 21). The UK public is very concerned about GM in the food chain, something they share with people across Europe according to EU surveys.

Recently, the major Swedish pig meat processor Scan announced it will stop accepting GM-fed pigs later this year. Food companies should also respond urgently to new research showing that intact GM Bt toxins have been detected in blood samples taken from women and umbilical cords in Canada. Due to be published in the peer-reviewed journal Reproductive Toxicology, the research challenges what biotech companies and regulators have always claimed that the Bt toxin in GM crops was destroyed in the gut and would not be absorbed into the blood supply.

Retailers should take the precaution of ensuring their food and feed supply chains are completely free of Bt crops until the Canadian research has been evaluated.

Pete Riley, campaign director, GM Freeze