Food Standards Agency (FSA) Chief Executive Catherine Brown has drawn vets and horse owners into the conspiracy behind horsemeat containing bute getting into the food chain.
Ms Brown highlighted that both had to sign horse passports if an animal was treated with the drug to ensure they were not sold on for human consumption.
"If both these people have done the right thing, horses with bute in don't make their way into the food chain," she said. "Someone has always broken the rules".
Previously the FSA said testing for bute is a "good indicator" of whether an animal was legally slaughtered.
When asked whether it would be fair to assume that six percent of the 9,000 horse carcasses she said were exported from Britain every year - which amounts to 540 animals - contained bute, Ms Brown replied, "That seems as reasonable a basis for making an estimate as any other".
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