In one sample from a Tesco beefburger, horsemeat accounted for approximately 29% of the meat content, the FSAI said.
Iceland said the FSAI's findings were concerning, stressing the company had "withdrawn from sale the two Iceland brand quarter pounder burger lines implicated in the study".
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.
BBC: Horsemeat row: Trade union's fears over Silvercrest jobs
"These products were sold to the catering and wholesale sectors and have been distributed to Ireland, the UK, Spain, France, Germany and The Netherlands, " the FSAI said.
Ms Blythman said the FSA compared very unfavourably to the Food Standards Authority Ireland (FSAI), which she said was "more pro-active" and unearthed the horsemeat contamination in the first place.
Meanwhile, the Irish Department of Agriculture and the Food Standards Authority of Ireland (FSAI) are currently working to develop testing protocols for DNA testing at meat processors in the Republic.
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".
Officials from the Republic of Ireland's food safety authority (FSAI) confirmed the meat had come from Liffey Meats and Silvercrest Foods, in Ireland, and the Dalepak Hambleton processing plant in Yorkshire.
Tests were carried out by food firm 3663 after McColgan's name appeared on a report by the Food Safety Authority of Ireland (FSAI) following an investigation which discovered horsemeat in some frozen burgers.
"3663 recognised a potential connection between a supplier of halal savoury pie products for the Ministry of Justice and one of their halal beef suppliers mentioned with the FSAI report, " the company said.
Tesco's group technical director, Tim Smith, said his company was informed of the test results by the FSAI on Tuesday and they "immediately withdrew from sale all products from the supplier in question".
The Irish Minister for Agriculture, Simon Coveney, said he was concerned by the FSAI's findings, and had sent government vets into the factory that produced the 29% horsemeat burger to interview management.
The Food Safety Authority of Ireland (FSAI) said the meat had come from two processing plants in the Irish Republic - Liffey Meats and Silvercrest Foods - and the Dalepak Hambleton plant in Yorkshire.
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.
BBC: Horsemeat row: Trade union's fears over Silvercrest jobs
The Food Safety Authority of Ireland (FSAI) said the contaminated meat had come from two processing plants in the Irish Republic - Liffey Meats and Silvercrest Foods - and the Dalepak Hambleton plant in Yorkshire.
The Food Safety Authority of Ireland (FSAI) said the contaminated meat had come from two processing plants in the Irish Republic - Liffey Meats and Silvercrest Foods - and the Dalepak plant in North Yorkshire.
One supplier, meat processor Silvercrest, based in County Monaghan, Ireland, lost supply contracts with Tesco, Aldi and Co-operative supermarkets after the Food Safety Authority of Ireland (FSAI) found horse and pig DNA in burgers it supplied.
On the evening on 15 January, the Republic's Food Safety Authority of Ireland (FSAI) announced the results of DNA tests it carried out on a number of beef products on sale in a number of supermarkets.
Initially the problem appeared to be confined to two processing factories in the Republic of Ireland and one in Yorkshire, but there was no risk to human health, as the FSAI and others were quick to point out.
Both ministers agreed that due to the close trading relationship between the two regions, the FSAI and the FSA would work to jointly agree an approach for protecting the authenticity of meat ingredients used in the manufacture of meat-based products.
The Food Safety Authority of Ireland (FSAI), which is conducting similar inquiries, said the meat had come from two processing plants in the Irish Republic - Liffey Meats and Silvercrest Foods - and the Dalepak Hambleton plant in North Yorkshire.
The FSAI tested 27 beef burgers and of that sample, 37% tested positive for traces of horse DNA, while 85% were found to contain traces of pig DNA. The amounts detected were very small, except for one beef burger from Tesco, that was found to contain 29% horsemeat.
In a statement, the company said it noted the FSAI's findings "with concern" and "would be working closely with its suppliers to investigate this issue and to ensure that all Iceland brand products meet the high standards of quality and integrity that we specify and which our customers are entitled to expect".
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