The Food Standards Agency (FSA) explored the idea of testing all horses slaughtered in the UK months before the horsemeat food crisis began in January.
But the agency was concerned that the costs of extra testing would cause every horse abattoir to close down.
Without the faster tests, it is likely that meat tainted with phenylbutazone continued to enter the food chain.
The agency says the testing was delayed to allow a comprehensive review of all the impacts.
The number of horses presented for slaughter in the UK has doubled in recent years, with more than 9,000 animals killed at the five approved abattoirs in 2012. Most of the meat was exported to Europe.
Animals that have been treated with the anti-inflammatory medicine phenylbutazone or bute are legally barred from entering the food chain. Details of any treatment with the drug are meant to be recorded in a passport document.
But the UK has had considerable problems with fraudulent passports due to the large number of organisations of varying quality licensed to issue the documents.
Last year the FSA was made aware that increasing numbers of horses with questionable passports were turning up at abattoirs. In the summer it implemented a series of extra tests to determine if bute-tainted meat was getting through.
The problem with testing for the drug is that it can take two weeks to get a result. By the time the agency knew the carcass was positive or negative, the meat had already been shipped on to markets across the EU.
In the UK there were five abattoirs approved for the killing of horses, including this one near Bristol
From its extra testing, the FSA learned that around 6% of horses were positive for bute, meaning that potentially hundreds of animals with the drugs in their system were going into the food chain.
The agency did introduce a "positive release system" - meaning the horsemeat was released only when certified drug-free on 11 February, once the crisis over horsemeat in beef products had begun.
But in emails released to the BBC under the Freedom of Information act, senior officials at the agency were shown to be looking into the costs of more rapid tests on 5 November, more than three months earlier.
So why was the more rapid system not put in place earlier?
"In any decision we could have made back in November we needed the evidence," Andrew Rhodes, the FSA's director of operations said.
"In November what you see in all the records is us seeking the evidence - the evidence we finished gathering early this year and we were able to make a decision on what we would do next and that was to introduce a positive release system."
'Questions to answer'
Labour shadow environment secretary Mary Creagh said the only reason the extra testing was finally introduced was because of the discovery of horsemeat in beef products. The real responsibility for the delayed testing, she said, lay with the government.
"We now know that in November plans were drawn up to test every horse slaughtered, yet nothing was done until after the horsemeat scandal hit the headlines," she said.
"Incompetent ministers have questions to answer about why these early warnings were ignored which led to bute-contaminated horsemeat being consumed."
The FSA rejects the idea that it introduced the positive release system only in response to the crisis - it said it was going to happen anyway.
It was concerned that the price of putting in place a 48-hour testing regime was very high.
Mr Rhodes said it was "more than likely" that the extra costs would have essentially closed down the slaughter industry.
"The average value of a horse carcass is anywhere up to £300, maybe a little more," he said.
"The test we were looking at, the time for the 48-hour turnaround was the same or more than the cost of the actual carcass and that would have caused problems in terms of passing on any cost.
"But what we have to remember is that we didn't have the evidence base back then to make the decision that we were able to make earlier this year."
The FSA was concerned that if abattoirs closed down, there would be a range of knock-on problems that would also have cost money to deal with.
"If we had stopped the industry operating then we might have seen all kinds of animal welfare issues as well as possible illegal slaughter and the trade going underground," Mr Rhodes added.
"That's not a good thing for consumers, it's much more important that we regulated the industry legally and that we find a way of making that work and that's what we've done."
Testing for bute is likely to continue for at least another year, he added, saying that there were still ongoing problems with the drug.
Mr Rhodes also said that owners and vets "aren't always signing [horses] out of the food chain when they should be, we see that in the investigations we carried out.
"Until that changes there will be a need to control the animals going into the food chain."