Upmarket supermarket Waitrose has become the latest retailer to pull beefburgers from its shelves in the horsemeat scare.
The company said it had taken frozen burgers made by Dalepak, one of the firms at the centre of the horsemeat contamination investigation, off sale "as a precaution" when it had its accreditation suspended.
Ten million burgers have been taken off supermarket shelves across Ireland and the UK as a result of the scandal when it was revealed some lines sold by Tesco, Aldi, Lidl and Iceland were discovered to have contained traces of horsemeat.
In a statement, Waitrose said its burgers had since been tested and were found to be 100% beef.
"As a consequence we are 100% confident in the integrity of our supply chain," it said.
"The ingredients in our burgers are simple with all meat traceable back to British farms that we know.
"Our technical team visited the Dalepak site last week and were happy that our products were produced to our high specification and separately from other companies' products (ours are produced at 6am before other any other burgers)."
The ABP Food Group, one of Europe's biggest suppliers and processors, stopped work at its Silvercrest Foods plant in Co Monaghan, Ireland, after tests last week revealed contamination in frozen burgers.
Tests had already shown that Silvercrest Foods and another of the company's subsidiaries, Dalepak Hambleton in Yorkshire, supplied beefburgers with traces of equine DNA to supermarkets, including one product classed as 29% horse.
On Thursday, Labour claimed a potentially carcinogenic drug might have entered the food chain through horse meat slaughtered in the UK.
Shadow environment secretary Mary Creagh told the Commons she had evidence that "several" horses slaughtered in the UK last year tested positive for the carcinogen phenylbutazone.
The Food Standards Agency (FSA) said it carried out checks in slaughterhouses for the drug and no animals found to contain it had entered the food chain.
It said: "In 2012 the FSA identified five cases where horses returned non-compliant results.
"None of the meat had been placed for sale on the UK market.
"Where the meat had been exported to other countries, the relevant food safety authorities were informed."
The FSA added: "During the recent horsemeat incident, the Food Safety Authority of Ireland (FSAI) checked for the presence of phenylbutazone and the samples came back negative."