The sedatives had been administered to three horses slaughtered in the UK and then exported, the British Agricultural Minister David Heath revealed according to a report from the AP news agency.
Observers in Sweden said they had not found traces of the drug.
“We are waiting for more test results but we don’t see any threat to the consumers,” Henrik Nyberg, Nordic head of Swedish ready-made meal producers Findus, told the Svenska Dagbladet (SvD) newspaper.
Officials in the UK, meanwhile, said the presence of the drug would likely not be harmful to humans.
“It is highly unlikely that a person who eats horsemeat with fenylbutazon would experiences any of the side effects associated with it,” national health service spokeswoman Sally Davies told AFP.
“A person would have to eat between 500 and 600 horsemeat burgers to get close to a daily dose for humans.”
On Wednesday night, Sweden’s Agricultural Minister Eskil Erlandsson met with his European counterparts in a bid to get to grips with the problem, which has been blamed on complicated supply chains, consumers’ low-price fixation, and little oversight from European Union level.
Meanwhile, the French company Spanghero, which is believed to have played a key role in the complicated supply chain, received an invoice for 42 tonnes of low-quality horsemeat, a revelation that contradicts the company’s own statement that it did not trade in “horsemeat of any kind”.
The invoice came from a Cyprus-based company believed to be selling meat from Romania.
Spanghero, which sold meat on to the Comigel food producers at the heart of the scandal, has yet to comment on the invoice, news agencies reported.