Plans are afoot to give greater protection to trafficked prostitutes working in the UK. But how big is the current problem? It's hard to know, although that hasn't stopped some people from thinking they do, writes Ruth Alexander, of Radio 4's More or Less.
Concern about trafficked sex workers has grown as borders between countries have become more porous. Now the government is planning to crack down on the problem by criminalising anyone caught paying for sex with a prostitute who has been trafficked or is marketed by a pimp.
But how many prostitutes are there in the UK who are "controlled for another person's gain"?
Groups such as the Poppy Project, which has carried out research into commercial sex, acknowledge that it is not possible to estimate the number of people who are trafficked into the sex industry. Although they suspect the numbers are growing.
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But that doesn't stop some people thinking they know.
Fiona Mactaggart, a Labour MP and former Home Office minister, is a supporter of the government's plans to change the laws on prostitution. In November, she told BBC Radio 4's Today in Parliament that "something like 80% of women in prostitution are controlled by their drug dealer, their pimp, or their trafficker."
In fact, it is impossible to find that number in any research done on this subject.
The Poppy Project found 81% of prostitutes working in London in 2004 were foreign nationals. But foreign doesn't mean forced.
Yet the two are often confused, says Julia O'Connell Davidson, professor of sociology at Nottingham University and an expert on trafficking and the global sex trade. Some academic and press reports imply that foreign sex workers are all being made to work against their own free will, she says.
'Don't match hype'
She and her team did some research, ringing up massage parlours and escort agencies in London, asking exactly what sort of services they offered and what nationality the workers were. It is significant, she notes, that in two-thirds of cases researchers were told the prostitutes would not offer anal sex.
Fiona Mactaggart used the 80% figure; Jacqui Smith says there are 80,000 prostitutes
"That suggests to me that more sex workers in indoor prostitution in London exercise more control over the details of their working practices than a lot of the commentators believe," says Ms O'Connell.
And she points to high-profile police operations, where the results, in her opinion, do not match the hype.
"In 2006, Operation Pentameter carried out 515 raids on indoor prostitution establishments in the UK and Ireland over four months. It resulted in the 'rescue' of 84 women and girls believed to have been trafficked.
"This was followed by Operation Pentameter 2 in 2007. Again, the results didn't match the hype. In total 822 premises were visited and 167 victims identified."
So that is more than 1,300 premises raided in total - specifically targeted by police as likely to be abusive - and around 250 women rescued. That suggests the proportion of women in forced trafficking situations, while disturbing, is much lower than 80%.
We do not even know for sure how many prostitutes there are working in the UK.
The consensus is about 80,000. That figure - recently used by Home Secretary Jacqui Smith in an interview about the proposed new law - comes from research done 10 years ago by Hilary Kinnell, when she was working for an organisation providing health services to sex workers.
Ms Kinnell contacted 29 projects that provided services for sex workers to ask how many prostitutes they were working with. She had 17 responses. The average number of prostitutes per project was 665. She then multiplied that figure by 120, the total number of projects on her mailing list, to get an estimation of the total number of prostitutes.
"That brought the total up to very close on 80,000, which is still being quoted," Ms Kinnell says. "And I find that quite bizarre really. The figure was picked up by all kinds of people and quoted with great confidence but I was never myself at all confident about it. I felt it could be higher, but it also could have been lower."
Ms Kinnell is the first to point out the possible problems with her method: the centres responding might be larger than most; some sex workers might use more than one centre, and some might not be on the radar at all.
Exact numbers difficult
Yet over time, these caveats have been forgotten. Only the number remains.
Ireland is highlighting the issue of trafficked women with a TV advert
So is Fiona Mactaggart sticking by her 80% figure?
"Inevitably it's very difficult to get exact numbers here," she says. "But it's a combination of studies, many conducted by universities and so on which are quoted in the Paying the Price Home Office document [a 2004 consultation paper on prostitution]."
Information from the UN suggests there is a "very large extent of trafficking", she says, and "that most of it is women or children, and that the experience of most women in prostitution is akin to that of being trafficked".
"It is very difficult to get accurate figures in something that is an illegal activity in many of its forms but nevertheless I think that all the studies that have been done point to the same direction."
The Home Office have told us that they do not endorse or use the figure that 80 per cent of prostitutes are controlled by others.