Millions of burgers have been withdrawn from Burger King and supermarkets, after horse DNA was found in meat during inspections of factories in Ireland.
Last week the Food Standards Agency (FSA) said that contaminated burgers could also contain a horse drug.
Of 8,426 horses killed for food in Britain last year only 145 were tested. Of these, five contained phenylbutazone. This is banned from the food chain because it can cause liver and bone-marrow disorders.
The FSA was set up in 2001 after fatal outbreaks of Ecoli poisoning. Since 2010 cuts to the FSA and Trading Standards officers mean that bad meat is less likely to be discovered.