The “filler product” found in contaminated burgers sold in supermarkets came from Poland and was a mixture of beef and horse offcuts, according to Irish authorities, the Food Standards Agency (FSA) has said.
The contaminated meat was in the form of blocks of frozen product from a Polish supplier that had been used for a year, FSA chief executive Catherine Brown told the Commons Environment Committee.
Investigations are going on into how long contaminated meat might have been in use, Ms Brown told MPs.
Asked how UK consumers could know if horse meat had not been in burgers "for months, if not years", Ms Brown replied: "We haven't (in the past) identified horsemeat in burgers as a likely significant risk in this country, and therefore it is possible... and that's why I'm saying that it's very important now that we get to the bottom of the Polish connection and the Irish investigation because it is possible that these burgers have been on sale in this country.
"The probable limit of possibility... is a year because it's been a year that this supplier has been supplying. And therefore when the Polish get to the bottom of this we will hope to know whether it's likely that this has been going on for a year."
Asked if the burgers posed a health risk, she replied: "There is no evidence at the moment that there has been any unsafe food produced.
"Clearly what we all rely on in terms of a safe food to eat is a fully functioning system of controls and a set of audits which check to all of our satisfaction that that robust set of controls are being consistently applied. So where you find a case where they haven't, it raises your general level of anxiety about the wider system.
"But, as I say, the tests that the Irish did indicate that there's nothing to be concerned about."
The meat was processed at the Silvercrest plant in County Monaghan.
Earlier in the day, Tesco announced that it had dropped Silvercrest as a supplier in the wake of the horse burger scandal over a "breach of trust".
The supermarket was forced to issue a public apology earlier this month after tests discovered traces of the animal in beef products.
Today Tesco accepted responsibility for the fiasco but appeared to pin much of the blame on its supplier.
It said Silvercrest failed to comply with company policy, sourcing meat from abroad.
It has now vowed to introduce a new DNA testing system designed to detect any "deviation from our high standards".
Tesco group technical director Tim Smith told the committee the retailer did not have a different set of standards for its "value" burgers - found to have been contaminated with horsemeat - than any of its other lines.
He told the committee: "You end up doing pretty much the same level of check whatever the product is.
"My reassurance to Tesco customers is pretty straightforward: the rigor, the surveillance, the quality checking, the auditing take no notice of whatever the price that that product is being sold at or the recipe."
Under repeated questioning from the MPs regarding where responsibility for where the contamination lay, health minister Anna Soubry said: "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.
"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."
In a separate development, The Co-operative Group announced that independent tests of its own-brand burgers supplied by Silvercrest had found traces of less than 1% horse DNA in three samples and 17.7% in one sample.
A Co-operative spokeswoman said: "Our decision to withdraw these products at the first opportunity and cease taking further product from this site has proven to be the correct course of action.
"Whilst there are no safety issues involved, it is now apparent that some of the withdrawn products have not met the high standards we and our customers expect. We apologise for this.
"We specify that all meat in our frozen burgers should be 100% British but we now strongly believe that some of the meat used to produce these burgers came from outside the UK and was not British in origin, and as a result we have taken the decision to delist Silvercrest as a supplier with immediate effect.
"In addition we are tightening our already stringent quality checks to ensure our products meet the high specifications that we set on behalf of our customers."