This week the Food Safety Authority of Ireland (FSAI) made a horrifying discovery — three meat processing plants had somehow allowed horse meat to get into beef burgers that were then sold in supermarket chains in the UK and Ireland.

The findings were pretty horrific – while low levels were found in most burgers, one sample from the huge British supermarket Tesco had 29% horse meat as opposed to beef.

Pig DNA was also found in the burgers.

The confusing thing is that this isn't simply a case of contamination. "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," Professor Alan Reilly, chief executive of the FSAI, told the Guardian.

So how did it get into the beef burgers? Various theories are now doing the rounds, including one about Ulster gangs forming horse smuggling rings and even the idea that the meat came from the US.

But it gets worse. The British equivalent of the FSAI, the Food Standards Authority (FSA), today admitted it had never, ever tested beef burgers for horse DNA. No one has any idea how long this has been going on.

Tim Lang, a professor of food policy at City University, London, told the Daily Telegraph: “It could have been going on for years but we wouldn’t know about it because we have never conducted tests."