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Food standards authorities in the Irish Republic are certain that horsemeat found in beefburgers came from Poland, the UK Food Standards Agency has said.

FSA boss Catherine Brown told the Commons environment committee that a mixture of "beef and horse offcuts" were contained in a frozen block of filler product imported from Poland.

The meat was processed at the Silvercrest plant in County Monaghan.

Investigations have begun in Poland to find out how beef and horse were mixed.

Meanwhile, FSA investigations are still ongoing into how traces of horse DNA came to be found in burgers produced at processing plants in the UK.

And in a separate development, the Co-operative Group revealed independent tests of its own-brand burgers supplied by Silvercrest found traces of less than 1% horse DNA in three samples, and more than 17% in one sample.

The affected products have been withdrawn from sale and the Co-Op has joined Tesco in "delisting" Silvercrest as a supplier.

Ms Brown told MPs the Food Standards Agency of Ireland (FSAI) investigation had found the Polish supplier was used by the Silvercrest facility for about 12 months.

Traces of both horse and pig DNA were found in value beefburgers sold in Iceland, Tesco, Aldi, Lidl and Dunnes Stores.

Asked by MPs how UK consumers could know if horse meat had not been in burgers "for months, if not years", Ms Brown replied: "We haven't [previously] identified horsemeat in burgers as a likely significant risk in this country... that's why 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.

"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."

Ms Brown said she would not "rule out or in" any prosecutions until all the investigations had been completed, insisting "everything is still very much on the table".

She added that none of the samples tested by the FSAI had "anything in them that was unsafe to eat" and insisted there was "no evidence" to suggest the food was unsafe for human consumption.

Earlier on Wednesday, Tesco announced it was dropping Silvercrest, part of the ABP Food Group, as a supplier in the wake of the horsemeat scandal.

It also announced it was to introduce a "comprehensive system of DNA testing across our meat products".

Speaking to the committee, Tesco group technical director Tim Smith said the supermarket had approved seven suppliers for use by Silvercrest for the production of Tesco-branded burgers.

"Silvercrest, for whatever reason, chose to use suppliers we had not approved and audited," he said.

"If somebody chooses to step outside of that process... for whatever commercial reason, then it's impossible to check a supplier in Poland who we don't know even exists."

He said 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.

"You end up doing pretty much the same level of check whatever the product is," he added.

The Silvercrest plant is currently closed. ABP Food Group's chief executive Paul Finnerty earlier said the firm had "let customers down" and apologised for the failures.

The results of FSAI tests, released to the UK FSA on 14 January, revealed some burgers made at the Silvercrest plant contained up to 29% horsemeat, constituting a "gross contamination".

Some 10 million burgers were withdrawn from sale across all of the supermarket chains implicated.

UK health minister Anna Soubry, who was also questioned by the committee, said it was not yet clear where the blame for the horsemeat contamination should lie.

"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," she said.

"Until we can establish all those facts we can't roll it back in order to find out where the responsibility lies."