Supermarket chain Tesco has apologised after it sold premium-priced "corn-fed" chicken that had not consumed enough maize to meet minimum requirements.
The supermarket giant said this was a "completely unacceptable" mistake caused by a farmer accidentally giving his chickens the wrong feed.
The retailer said the error only lasted a short time, but could not immediately confirm whether any birds from the affected batch were still on sale at its stores.
Tesco issued the apology in response to tests carried out for The Times newspaper on corn-fed chicken sold by six supermarkets and upmarket department store food halls.
Scientists at the Department for Environment, Food and Rural Affairs (Defra) Central Science Laboratory in York found the Tesco chicken "was not in keeping with EU regulations".
All the other samples tested were found to have consumed enough maize to justify being sold as corn-fed.
A Tesco spokesman said: "The integrity of our food is our greatest concern at Tesco.
"We conducted an immediate investigation and we have already determined that for a short time the farmer concerned inadvertently fed standard free-range feed - which does contain corn — to his chickens instead of the free-range feed that contains sufficient corn to satisfy Defra standards.
"While this only happened for a short period of time, it is completely unacceptable to us and to our customers. We apologise for what appears to be a case of human error at a supplier that services different retailers."
The retailer reassured shoppers that there were no health risks associated with the mistake and pledged to carry out more frequent checks on its suppliers.
PA