The slaughterhouse in Yorkshire and the production plant in Wales were visited by the Food Safety Agency as authorities in 18 countries seek to find the source of the contaminated consignments.
Sweden became the latest country to begin DNA testing products on its shelves.
The moves happened amid suggestions that up to 70,000 horses in Northern Ireland have disappeared in recent times and there was evidence of a large trade in undocumented animals to Britain.
The British Labour Party’s Mary Creagh said there was clear evidence of an illegal trade in horse meat.
Ms Creagh said there were 70,000 horses unaccounted for in Northern Ireland and unwanted animals had been given false paperwork before being sold for €10 and then resold to meat dealers for €500.
“The Ulster Society for the Prevention of Cruelty to Animals have clear evidence of an illegal trade of unfit horses from Ireland to the UK for meat, with horses being re-passported to meet demands for horse meat in mainland Europe,” she said.
She also said it had been convenient to blame Poland and Romania and overlook problems in this part of Europe.
However the deputy chief veterinary officer in the North, Robert Huey, said his office did not know the basis for the 70,000 horse claim.
“I do not know what the basis of it is.” And he said there had been no evidence of illegal or out-of-hours activity at slaughterhouses.
The comments came as officials in the North said all evidence gathered so far pointed to fraudulent activity in the meat trade.
In recent days evidence has emerged of horses being kept without passports in this country. This is illegal.
However, the large difference between the price of beef and horse meat was also cited as a reason why criminals would seek to compromise the system.
Gerry McCurdy, director of the Food Standards Agency in Northern Ireland, said there were gains to be made in replacing Irish beef for cheaper meats.
“If you are selling horse meat, someone said for £700 a tonne versus £3,000 a tonne, for anyone who is in the game of being fraudulent there is an extremely helpful financial incentive there for them to break down product,” he said.
Mr McCurdy was appearing before a Stormont committee that was inquiring into the discovery of horse meat in a consignment of beef that had been stored by another company at the Freeza Meats premises in Newry.
Mr McCurdy said there was more demand for cheaper processed meat.
“The local beef is probably too expensive for those burgers. That still does not excuse supermarkets from ensuring that food is not safe, simply because it is the low end of the market in that sense should still not prejudice consumers,” he added.
He said the information officials possessed would direct them towards some form of fraudulent activity, but it was unclear where that took place.
“The information available to us does point in the direction of Europe but we cannot be definitive about that at this point in time until those investigations are complete,” he added.