Police and health workers say there is a "real problem" in Wales of trafficking women for prostitution.
Police said gangs were making 'significant' money from prostitution
Two men were jailed in December for being part of a gang who "bought" a Lithuanian woman in London and forced her to work as a prostitute in Cardiff.
Police said afterwards that they were examining more cases where women had been forced into the sex industry.
BBC Wales' Week in Week Out programme has uncovered further examples of foreign women forced into prostitution.
The programme talks to one eastern European woman who describes how she came to the UK after being promised a job in a pub.
But after arriving in Britain, she said she was "bought" for £4,500, then beaten, raped and forced to sell herself for sex.
Giles York said prosecutions were under way
The woman, who has not been identified, told Week in Week Out: "I was shocked when he beat me.
"They told me you are going to work but because I started shaking and was so scared they said leave her for three days to relax her because she is going to scare the customers."
The woman ran away from the gang, but has continued to work as a prostitute in Cardiff.
Outreach workers who provide advice and medical help to prostitutes in the Cardiff area said the extent of people trafficking to the city was beginning to emerge.
Consultant clinical psychologist Richard Pates, who set up an outreach programme for prostitutes in Cardiff said: "We think of the abolition of slavery more than 150 years ago, and yet this is what these women are ending up as being, they are ending up as being slaves."
Dr Pates said that of more than 50 prostitutes his outreach colleagues saw every week, about 40% were foreign and many admitted to being trafficked.
Gjergj Mungiovi-Cuka (left) and Akil Likcani were jailed last year
He said: "It's very hard to quantify the numbers, because it is obviously a hidden trade, it's a hidden profession.
"But if I asked our outreach worker to take you out today, she would have no trouble in introducing you to 20 women that would probably have been trafficked, so it is not difficult to find women if you know where you are looking.
"It is a real problem in this city and in Wales." Police working to combat sex trafficking told the programme that organised gangs were making "millions" of pounds from prostitution.
Giles York, assistant chief constable of South Wales Police, said: "I have covert operations running, testing the stereotypical places to see if there is prostitution happening there, to see if there are people being exploited there.
"I also have post-prosecution cases running where we are taking away the assets from those organised criminals who have benefited from this criminality.
Richard Pates said women were ending up as 'slaves'
"And this is significant money, we are talking of money up to the millions that these people are able to bank and exploit when they have a whole network of brothels working to them."
The Week in Week Out investigation follows a court case in December last year where Gjergj Mungiovi-Cuka, 19, was found guilty of being part of a gang who paid £5,000 for a Lithuanian woman and forced her to work as a prostitute in Cardiff.
He was sentenced to four years' custody at Cardiff Crown Court, while his accomplice, Akil Likcani, received six years after admitting trafficking and controlling a prostitute.
A third gang member - known as Benny - remains at large. The woman, who was 20 at the time, had been forced to work in three Cardiff brothels.
Judge Phillip Richards told the two men: "Both of you were involved in an evil trade which has been described as the 21st century's slavery."
Week in Week Out is broadcast at 2235 GMT on BBC1 Wales on Tuesday and at 2130 GMT on BBC2W on Wednesday.