Traces of horse and pig DNA were discovered in burger products sold in some of the biggest supermarkets in the United Kingdom and Ireland, the Food Safety Authority of Ireland announced Tuesday.

The improper meat was also sold at global retailer Tesco and most of the tested burgers had very small traces of animal meat besides those of cows.

In nine of the 10 burger samples taken from the four retailers, and from the Irish chain Dunnes Stores, horse DNA was found at very low levels. However, in one sample, Tesco Everyday Value Beef Burgers, the level of positive DNA indicated horse meat accounted for 29 percent relative to the beef content. Many of them were also found to contain pig DNA.

The blame was placed on a meat processor in northern Ireland by Ireland's Agriculture Minister Simon Coveney, who said that was where the imported additive came from, according to The Huffington Post.

Coveney stated that the additive was "either falsely labeled, or somebody made a mistake, or somebody was behaving recklessly. That allowed some horsemeat product to come into the system that shouldn't have been here."

Prime Minister David Cameron addressed the issue to Parliament on Wednesday and urged that further investigations be conducted by Britain's Food Standards Agency.

"People in our country will have been very concerned to read this morning that when they thought they were buying beef burgers they were buying something that had horse meat in it," Cameron stated, according to Reuters. "This is a completely unacceptable state of affairs."

The results of the tests started a new investigation into meat processing plants in the U.K.

Tesco released a statement shortly after the results were announced that stated the following: "We are working with the authorities in Ireland and the U.K., and with the supplier concerned, to urgently understand how this has happened and how to ensure it does not happen again."

Tesco also responded by pulling from its shelves all products from the company that had supplied the dubious burgers.