The supermarket giant said this was a "completely unacceptable" mistake caused by a farmer accidentally giving his chickens the wrong feed.

The retailer later confirmed that no chickens from the batch affected by the mistake were now on sale at any of its supermarkets.

Tesco issued the apology in response to tests carried out for The Times 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", the paper reported. All the other samples tested were found to have consumed enough maize to justify being sold as corn-fed.

According to Defra's website, the term "corn-fed" is generally understood to mean that the feed formula given during the greater part of the chicken's fattening period must contain at least 50% maize.

Corn-fed chicken, which has a distinctive yellow colour, sells at a premium but has become increasingly popular in recent years as discerning consumers seek out better quality produce.

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."