The Food Standards Agency (FSA) has revealed that burgers contaminated with horsemeat may have been on sale for up to a year.
Earlier this month, Tesco and a number of other supermarkets removed certain brands of frozen beef burgers from their shelves after a study revealed they contained the horsemeat.
The contaminated meat was processed at the Silvercrest plant in County Monaghan, but it is believed the horsemeat came from a plant in Poland.
In questioning by the Commons Environment Committee, FSA chief executive Catherine Brown said the agency is investigating how long the contaminated meat might been in use but the estimate was about 12 months.
"The probable limit of possibility ... is a year because it's been a year that this supplier has been supplying," she said.
Ms Brown also said that Irish authorities believed that "filler product" found in the contaminated burgers came from Poland and was a mixture of beef and horse offcuts.
Asked if the burgers posed a health risk, Ms Brown said: "There is no evidence at the moment that there has been any unsafe food produced."
Earlier, Tesco said it had dropped Silvercrest as a supplier following an investigation into why the horsemeat was used in its products.
The supermarket giant's technical director, Tim Smith, said that new DNA testing would be introduced for all of its meat products to avoid similar mistakes happening again.
"The evidence tells us that our frozen burger supplier, Silvercrest, used meat in our products that did not come from the list of approved suppliers we gave them," he said.
"Nor was the meat from the UK or Ireland, despite our instruction that only beef from the UK and Ireland should be used in our frozen beef burgers.
"Consequently we have decided not to take products from that supplier in future. We took that decision with regret but the breach of trust is simply too great."
Mr Smith added: "Ultimately Tesco is responsible for the food we sell, so it is not enough just to stop using the supplier.
"To underpin the strong measures already in place, we will now introduce a comprehensive system of DNA testing across our meat products. This will identify any deviation from our high standards."
Silvercrest said that it "never knowingly bought or processed horsemeat", and insisted that all of its purchases were from approved and licensed EU plants.
Health Minister Anna Soubry also faced repeated questioning from the MPs regarding where responsibility for where the contamination lay.
"We don't know whether or not the Irish, the people that made the burgers, didn't themselves know that the meat coming in was in some way contaminated. We don't know that yet," she said.
"So therefore it could be that there is a genuine fault in Poland with the particular supplier of this meat, either deliberately or not deliberately because they haven't been doing the right checks.
"And until we can establish all those facts we can't roll it back in order to find out where the responsibility lies."
The study examining the authenticity of a number of beef burgers, beef meals and salami products available from retail outlets in Ireland found horsemeat accounted for approximately 29% of the content in one sample of Tesco Everyday Value Beef Burgers.
The findings sparked a public outcry and 10 million burgers were taken off shelves.