British Prime Minister David Cameron weighed in today on the discovery that some hamburger products sold in supermarkets in Ireland and the United Kingdom have been found to contain horse meat, calling it "completely unacceptable."

Tests on beef products sold in stores found low levels of horse DNA in hamburgers, and while there is no health risk, stores are pulling implicated products from their shelves.

Most of the products include trace amounts of the horse DNA (about 0.1% - 0.3%), but one product, a beef burger sold at Tesco, was found to contain 29 percent horse meat.

"It is extremely disturbing news," Cameron said at a meeting in parliament today. "I have asked the Food Standards Agency to conduct an urgent investigation into this."

It is unclear how the horse meat ended up in the beef products. Several of the products also contain pig DNA, but experts say that isn't out of the ordinary.

"Whilst there is a plausible explanation for the presence of pig DNA in these products due to the fact that meat from different animals is processed in the same meat plants, there is no clear explanation at this time for the presence of horse DNA in products emanating from meat plants that do not use horse meat in their production process," Prof. Alan Reilly, chief executive of the Food Safety Authority of Ireland (FSAI) said to the Press Association.

Although horse meat is consumed in other parts of the world, including France, Mexico, and most commonly in central Asia. However, in much of the Western world, horses are more commonly considered pets, or are used for work or recreation.

"In Ireland, it is not in our culture to eat horse meat, and therefore, we do not expect to find it in a burger," Reilly said.

"It is an extremely serious issue," Cameron said today. "People in our country would have been very concerned to read this morning that when they thought they were buying beefburgers, they were buying something that had horse meat."

Consumers can return any of the products found to have horse DNA, and affected stores are taking the products off their shelves and say they are working with suppliers to determine the cause.

Prime Minister Cameron said today the Food Standards Agency is beginning an investigation today.

"They will be meeting retailers and processors this afternoon," Cameron said. "They will be working with them to investigate the supply chain, but it is worth making the point that retailers have to be responsible for what they sell and where it has come from."

Ireland's food exports surpassed 9 billion euro for the first time last year, with meat and livestock as the strongest sector.