The European Union's law enforcement agency was sent evidence from two sites in Tottenham and one in Hull which were investigated last week.
Three men arrested by police have been released on bail pending further inquiries. Dafydd Raw-Rees, 64, the owner of Farmbox Meats near Aberystwyth, and a 42-year-old man, were arrested in Wales on Thursday on suspicion of offences under the Fraud Act. A 63-year-old man was arrested at Peter Boddy Slaughterhouse in Todmorden, West Yorkshire.
Catherine Brown, chief executive of the Food Standards Agency (FSA) said while tests provided a "snapshot", they could not establish how long the adulterated meat scandal had been going on or the number of batches affected. "I think it is unlikely we will ever know that," she said. "It is shocking".
Test results for possible horse meat contamination released by the FSA on Friday show just over 1% of around 2500 beef products had positive results relating to seven different lines, which have already been withdrawn from sale.
The head of one major UK supermarket chain insisted that the horse meat scandal was not "the tip of an iceberg" and that supermarkets had not been slow to react.
However, Justin King, chief executive of Sainsbury's, conceded there was some way to go before the food industry could fully explain how the crisis had come about.
He added: "I think there are some encouraging signs from today's tests that we are starting to get to the bottom of this particular issue.
"We in Sainsbury's have a huge testing programme. We have 50-odd people whose full-time jobs are to test product."
Scotland's Rural Affairs secretary said he had spoken with retailers last week to urge them to "reconsider their sourcing policies".
Richard Lochhead said recent events across Europe were "completely unacceptable", adding that retailers' response to the issue had been "frustrating" and raised serious questions.
He said: "Of course testing is the priority, but it is also the very least they should be doing. They must also be reflecting on their sourcing and purchasing policies."
He said companies supplying meals to the public sector in Scotland – including to schools, hospitals and prisons – have been carrying out extensive checks.
"Currently there is absolutely no evidence to suggest this is an issue in Scotland," he said.
The products linked to the positive test results: Aldi special frozen beef lasagne and spaghetti bolognese, Co-op frozen quarter pounder burgers, Findus beef lasagne, Rangeland catering burger products, and Tesco value frozen burgers and value spaghetti bolognese.