London: Investigations are continuing into how horse DNA contaminated beefburgers made at a British plant, after Irish authorities confirmed that Polish suppliers were responsible for equine material found in Tesco and other products.

The UK Food Standards Agency said inquiries concentrated on supplies sent to the Dalepak Hambleton plant in North Yorkshire before last October. Tests on samples collected this month were negative.

The agency did not know if raw ingredients used at Dalepak were from the same suppliers as those identified by the Irish agriculture minister, Simon Coveney, on Saturday as being responsible for contamination of burgers made at Silvercrest in County Monaghan, one of Europe’s biggest burger plants and responsible for making 200 million burgers a year, including Tesco’s.

Silvercrest and Dalepak are owned by ABP Foods. They and another Irish plant, Liffey Meats, were implicated in the scare earlier this month caused by food standards checks late last year. Follow up tests on burger samples from Liffey Meats made by the Irish government gave that plant the all-clear last week.

Products made for Tesco, Lidl, Aldi and Iceland were found to have horse DNA in the initial scare although it was the 29 per cent equine DNA in one sample from a Tesco burger made at Silvercrest that sparked further investigations by Irish and UK authorities. Other companies whose burgers were not tested also withdrew products as a precaution.

Burger King stopped using burgers made at Silvercrest, which suspended production more than a week ago and is now being deep cleaned.

More than 10 million beefburgers are thought to have been removed from sale because of the scare, although authorities in the UK and Ireland have repeatedly said they posed no threat to human health.

In an interview with RTE in Ireland on Sunday, Coveney said the contamination of raw materials, by up to 20 per cent equine DNA content relative to beef according to the latest samples, “should not have happened”. ABP now “had a job” to rebuild trust with big customers, and he had talked at length “at a very senior level” with Tesco and Burger King. All products now held at Silvercrest were being destroyed or put in cold store. Ingredients from Ireland and the UK had not been contaminated, said Coveney.

“Now we can be very credible as to where this problem came from,” he said. “My understanding is that the company involved takes beef product from about five different slaughtering facilities in Poland.” He accepted that Europe’s “very good regulatory system” for food safety and traceability had not worked in this case.

In all, about 150 samples of primary products and ingredients related to Silvercrest were tested as authorities worked round the clock for 10 days after theFood Safety Authority of Ireland (FSAI) first raised the alarm. Tesco said the source of horse DNA identified by the Irish government correlated with the results of its own investigations at the plant. It would give detailed consideration to all the findings. The supermarket, which apologised to customers after the original contamination, had to apologise again last week when burgers that were supposed to be off the shelves were found on sale in Oxford.

Waitrose, among the companies with burger lines made at Dalepak, where production was not suspended, has decided to restock its shelves.

Sainsbury’s, also not implicated, said it would not do so until it had completed its own investigation. ABP, which has its own food plant in Poznan, Poland, said this was not the source of the contaminated meat and none of its plants processed horsemeat. There would be new management at Silvercrest and the company had started a new DNA testing regime for contamination by equine and other sources.