Operation Pentameter, launched by the Home Office in 2006, aimed to free an estimated 4,500 victims of trafficking forced to work in the sex industry.
It was also designed to catch the criminals responsible for exploitation.
But a BBC Wales Week in Week Out investigation has revealed serious concerns that it is not delivering.
Operation Pentameter One resulted in 88 victims being rescued but Vernon Coaker MP, a Home Office minister, admitted to programme-makers he does not know where the vast majority of those are.
“I don’t know where the 76 [of the 88] all actually are but what I do know is that they will have been dealt with in the appropriate way,” he said.
He said he did not know how much public money had been spent across Wales and the rest of the UK on Pentameter One or Pentameter Two – which followed on a year later.
Nor could he tell the programme how many victims had been rescued in total.
The admission comes in the wake of a high-profile court case last week involving brothel boss Diana Jones, who ran a string of illegal operations in Cardiff and Swindon.
She was prosecuted as part of Pentameter, but there was no evidence of trafficking and the judge said no one at her brothels had been harmed.
In the Week In Week Out programme broadcast on Monday evening, she gave an insight into the illegal operation which police say turned over nearly £3m.
The programme reveals how it was business as usual throughout the investigation.
The court heard the brothels had run with at least some police officers knowing what went on behind closed doors – something the force had contested in court.
When challenged in the programme about whether South Wales Police officers had “turned a blind eye” to Diana Jones’s illegal operations, assistant chief constable Giles York said: “I do not know what is the minds of my officers, but what I can tell you is that she has been prosecuted now and it was a successful prosecution and she’s pleaded guilty to it.”
He said he believed Pentameter had been a success in his force area with five victims rescued, and nearly £500,000 recovered from offenders.
He said his officers would continue to pursue Diana Jones for the money she made from her illegal activities.
She was given a suspended prison sentence for brothel keeping and faces financial ruin.
She was investigated after taking two trafficked eastern European women to police for safety.
“I think Pentameter had all the right reasons for starting the operations off but in terms of success, and capturing traffickers, no, they got far more success from people in the sex industry, in the know, passing information on,” she told the programme.
Week In Week Out’s Behind Closed Doors programme is on BBC One Wales at 2030 GMT on Monday.