An independent veterinary committee had "repeatedly expressed concern" about a drug found in UK horsemeat destined for export, the BBC has learned.
The discovery of horsemeat in UK foodstuffs is raising big concerns that UK testing regimes are not sufficient.
There are worries that if unregulated horsemeat is substituted for beef it could expose people to a drug called phenylbutazone - often called "bute".
Once used as an anti-inflammatory, its toxicity to some people led to a ban.
The drug in rare cases caused a serious blood disorder known as aplastic anaemia.
But it is still used widely to treat horses. Once treated these animals are not supposed to enter the food chain.
In the UK about 8,000 horses a year are slaughtered for human consumption. This meat is then exported to other European countries. Under EU regulations, it must be tested for a range of substances including bute.
The independent committee said that it had "repeatedly expressed concern over residues of phenylbutazone entering the food chain".
"The number has gone up a little over the past three or four years," committee chairman Dr Dorothy Craig told BBC News.
"We're finding a rate of about 5%. It's banned, so the number of non-compliant samples should be zero."
Since 2005, horses are required by law to have a "passport" that contains a declaration as to whether the horse is intended for human consumption.
These passports are also used to record if an animal has been treated with bute; most of the UK positives have come from difficulties with these documents.
"That's really where the problems come from - either a genuine error or where there's deliberate fraud going on," said Dr Craig.
In June 2012, the European Commission's Directorate General for Health and Consumers issued its summary of an audit of abbatoirs in Italy - the EU's biggest consumer of horsemeat. In it, they noted "numerous shortcomings were detected in the passports".
But the global nature of the horsemeat business is also causing problems in tracing exposure to bute.
Using bute on horses for human consumption is banned in the EU, but thousands of tonnes of horsemeat is imported from the US, Canada and Mexico where practices are different.
Many of the animals killed for food in these countries were once racehorses, and the use of bute at racetracks across the US is so widespread that one scientist speaking to the New York Times called these horses "walking pharmacies".
Research published in the journal Food and Chemical Toxicology in 2010 highlighted this risk, and EU officials warned last year about serious problems in verifying whether horses killed in Mexico were drug-free.
"This is a very big trade," says Mark Jones from Humane Society International.
"The potential is there for quite significant contamination and residues, given that the route by which this meat is moving is very far from watertight."