FSA Chairman Catherine Brown told members of Parliament a Polish supplier implicated in the food scandal supplied beef "filler" to Silvercrest, a meat processor in County Monaghan County, Ireland, since at least 2011, The Daily Telegraph reported.
The uncooked beefburgers were sold to food companies including Tesco, which pulled all products supplied by the Irish company.
Fast-food giant Burger King stopped using Silvercrest productions although tests showed no horse DNA in samples of their burgers.
"How do we know that this horse meat has not been in beef burgers for months if not years?" MP Anne McIntosh asked Brown.
"It is possible that these burgers have been on sale in this country for a year," Brown replied. "This filler had been used for a year."
Testing of the burgers showed at least 20 percent of the DNA was horse.
Brown said investigators in Ireland and Poland hope to trace the origin of the horsemeat.