Three men were arrested tonight by officers investigating the horse meat scandal.
One of the men was arrested at a plant in West Yorkshire, the other at a side in Aberystwyth. Both were inspected on Tuesday by the Food Standards Agency.
The two plants became the first UK suppliers suspected of passing off horse meat for beef.
Production at both plants was suspended pending the outcome of investigations into claims they supplied and used horse carcasses in meat products purporting to be beef for burgers and kebabs.
The FSA said on Tuesday it had "detained" all meat found at the premises and seized paperwork and customer lists from the two companies.
The arrests came as it emerged a significant amount of horse meat containing the painkiller phenylbutazone - or "bute" - could have been entering the food chain for some time.
Authorities in Britain and France are trying to trace the carcasses of six horses contaminated with bute - which were slaughtered in a UK abattoir and may have entered the human food chain across the Channel.
The drug, which is potentially harmful to human health, was detected in eight horses out of 206 tested by the FSA in the first week of this month.
Two were intercepted and destroyed before leaving the slaughterhouse but the other six were sent to France, where horse meat is commonly eaten.