There is something familiar about the tide of misinformation which has swept through the subject of sex trafficking in the UK: it flows through exactly the same channels as the now notorious torrent about Saddam Hussein's weapons.

In the story of UK sex trafficking, the conclusions of academics who study the sex trade have been subjected to the same treatment as the restrained reports of intelligence analysts who studied Iraqi weapons – stripped of caution, stretched to their most alarming possible meaning and tossed into the public domain. There, they have been picked up by the media who have stretched them even further in stories which have then been treated as reliable sources by politicians, who in turn provided quotes for more misleading stories.

In both cases, the cycle has been driven by political opportunists and interest groups in pursuit of an agenda. In the case of sex trafficking, the role of the neo-conservatives and Iraqi exiles has been played by an unlikely union of evangelical Christians with feminist campaigners, who pursued the trafficking tale to secure their greater goal, not of regime change, but of legal change to abolish all prostitution. The sex trafficking story is a model of misinformation. It began to take shape in the mid 1990s, when the collapse of economies in the old Warsaw Pact countries saw the working flats of London flooded with young women from eastern Europe. Soon, there were rumours and media reports that attached a new word to these women. They had been "trafficked".

And, from the outset, that word was a problem. On a strict definition, eventually expressed in international law by the 2000 Palermo protocol, sex trafficking involves the use of force, fraud or coercion to transport an unwilling victim into sexual exploitation. This image of sex slavery soon provoked real public anxiety.

But a much looser definition, subsequently adopted by the UK's 2003 Sexual Offences Act, uses the word to describe the movement of all sex workers, including willing professionals who are simply travelling in search of a better income. This wider meaning has injected public debate with confusion and disproportionate anxiety.

Two academics from the University of North London, Liz Kelly and Linda Regan, tried to estimate the number of women who had been trafficked in the UK during the calendar year 1998, an exercise which they honestly described as "problematic".

First, there was the problem of the word, which Kelly and Regan solved by accepting all variations of its meaning. Then, there was the shortage of facts. They spoke to specialists, studied news reports and surveyed police, who reported that 71 women had been "trafficked", whether willingly or not, during 1998. In Stopping Traffic, which they published in May 2000, Kelly and Regan argued that the real scale of the problem was probably bigger than this and, in the absence of any accurate data, they made various assumptions which they themselves described as "speculative".

At the very least, they guessed, there could be another 71 trafficked women who had been missed by police, which would double the total, to 142. At the most, they suggested, the true total might be 20 times higher, at 1,420.

But reaching this figure involved a further quadrupling of the number of victims missed by police, plus quadrupling existing estimates by sex health workers, plus assuming the accuracy of a newspaper report that "hundreds" of women had been trafficked into the UK from Albania and Kosovo, plus assuming that mail-order brides were also victims of trafficking, plus adding women who were transported within the UK as well as those brought into the UK.

Kelly and Regan were transparent and honest about the speculative character of their assumptions. They were clear about their adoption of the widest possible meaning of the term. They presented their conclusion with caution: "It can be estimated that the true scale of trafficking may be between two and 20 times that which has been confirmed."

And they presented their conclusion as a range of possibilities: "It is recognised that this is a wide range, but it indicates the likely scale of the problem while reflecting the poverty of information in this area."

During the following years, the subject attracted the attention of religious groups, particularly the Salvation Army and an umbrella group of evangelicals called Churches Alert to Sex Trafficking Across Europe (Chaste). Chaste explicitly campaigned for an end to all prostitution and, quoting their commitment to the principles of the Kingdom of God, they were enlisted as specialist advisers to the police.

Chaste took the work of Kelly and Regan, brought the estimate forward by two years, stripped out all the caution, headed for the maximum end of the range and declared : "An estimated 1,420 women were trafficked into the UK in 2000 for the purposes of constrained prostitution."

The misleading figure was repeated in news stories and adopted by politicians. Even the government's Crimestoppers campaign recycled it. And over and over again, the absence of a definition in the original work was replaced with the certainty that this was about women who were forced to work against their will. Chaste spoke repeatedly about "sexual enslavement" and "sex slavery".

Three years after the Kelly/Regan work was published, in 2003, a second team of researchers was commissioned by the Home Office to tackle the same area. They, too, were forced to make a set of highly speculative assumptions: that every single foreign woman in the "walk-up" flats in Soho had been smuggled into the country and forced to work as a prostitute; that the same was true of 75% of foreign women in other flats around the UK and of 10% of foreign women working for escort agencies. Crunching these percentages into estimates of the number of foreign women in the various forms of sex work, they came up with an estimate of 3,812 women working against their will in the UK sex trade.

Margin of error

The researchers ringed this figure with warnings. The data, they said, was "very poor" and quantifying the subject was "extremely difficult". Their final estimate was "very approximate", "subject to a very large margin of error" and "should be treated with great caution" and the figure of 3,812 "should be regarded as an upper bound".

No chance. In June 2006, before the research had even been published, the then Home Office minister Vernon Coaker ignored the speculative nature of the assumptions behind the figure, stripped out all the caution, headed for the maximum end of the range and then rounded it up, declaring to an inquiry into sex trafficking by the Commons joint committee on human rights: "There are an estimated 4,000 women victims."

The Christian charity Care announced: "In 2003, the Home Office estimated there were 4,000 women and girls in the UK at any one time that had been trafficked into forced prostitution." The Salvation Army went further: "The Home Office estimated that in 2003 ... there were at least 4,000 trafficked women residing in the UK. This figure is believed to be a massive underestimation of the problem." Anti-Slavery International joined them, converting what the Home Office researchers had described as a "very approximate" estimate into "a very conservative estimate".

The Home Office, at least, having commissioned the research, was in a position to remind everybody of its authors' warnings. Except it didn't.

In March 2007, it produced the UK Action Plan on Human Trafficking and casually reproduced the figure of 4,000 without any of the researchers' cautions.

The evidence was left even further behind as politicians took up the issue as a rallying call for feminists. They were led by the Labour MP for Rotherham and former Foreign Office minister Denis MacShane, who took to describing London as "Europe's capital for under-aged trafficked sex slaves". In a debate in the Commons in November 2007, MacShane announced that "according to Home Office estimates, 25,000 sex slaves currently work in the massage parlours and brothels of Britain."

There is simply no Home Office source for that figure, although it has been reproduced repeatedly in media stories.

Two months later, in another Commons debate, MacShane used the same figure, but this time he attributed it to the Daily Mirror, which had indeed run a story in October 2005 with the headline "25,000 Sex Slaves on the Streets of Britain." However, the newspaper had offered no evidence at all to support the figure. On the contrary, the body of its story used a much lower figure, of between 2,000 and 6,000 brought in each year, and attributed this to unnamed Home Office officials, even though the Home Office has never produced any research which could justify it.

MacShane was not deterred.

"I used to work for the Daily Mirror, so I trust the report," he said.

Sources

The then solicitor general, Vera Baird, replied by warning MacShane that "we think that his numbers from the Daily Mirror are off" and then recycled the figure of 4,000 without any of the researchers' cautions. MacShane then switched line and started to claim, for example in a letter to the Guardian in September 2008, that there were "18,000 women, often young girls, trafficked into Britain as sex slaves." He used this same figure in another debate in the House of Commons, adding "We have to get the facts and figures right."

On this occasion, the source he was quoting was Pentameter Two, the six-month national police operation which failed to find a single person who had forced anybody into prostitution. But MacShane had a point: presenting the results of the operation to the press in July 2008, its operational head, Tim Brain, the chief constable of Gloucester, was widely reported to have said that there were now 18,000 victims of trafficking in the UK and that this included under-age girls.

Other senior figures who were involved with this press conference say they were taken completely by surprise by Brain's claim. "None of us knew where that came from," according to one senior figure. "It wasn't in his pre-brief. It wasn't in anything: ministers weren't briefed. Tim may have meant to say 1,800 and just got his figures mixed up."

Brain now agrees that the figure is not correct and suggested to the Guardian that he had been trying to estimate the total number of prostitutes in the UK, not the total number of trafficked women.

But the damage had been done. Patrick Hall, Labour MP for Bedford, solemnly told the House of Commons that there was sex trafficking "in towns and villages throughout the land."

Fiona Mactaggart, a former Home Office minister, in January 2008 outstripped MacShane's estimates, telling the House of Commons that she regarded all women prostitutes as the victims of trafficking, since their route into sex work "almost always involves coercion, enforced addiction to drugs and violence from their pimps or traffickers." There is no known research into UK prostitution which supports this claim.

In November 2008, Mactaggart repeated a version of the same claim when 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." Again, there is no known source for this.

Challenged to justify this figure by a different Radio 4 programme, More or Less, in January 2009, Mactaggart claimed that it comes from the Home Office's 2004 report on prostitution, Paying the Price. But there is no sign of the figure in the report.

In the summer of 2004, The Poppy Project, which is committed to ending all prostitution on the grounds that it "helps to construct and maintain gender inequality", surveyed London prostitutes working in flats and found that 80% of them were foreign, a finding which is well supported. They then added, without any clear evidence, that "a large proportion of them are likely to have been trafficked into the country", a conclusion which is challenged by specialist police, but which was then recycled through numerous media reports and political claims.

Last year (2008), Poppy published a report called The Big Brothel, which claimed to be the most comprehensive study ever conducted into brothels in the UK and which claimed to have found "indicators of trafficking in every borough of London".

That report was subsequently condemned in a joint statement from 27 specialist academics who complained that it was "framed by a pre-existing political view of prostitution". The academics said there were "serious flaws" in the way that data had been collected and analysed; that the reliability of the data was "extremely doubtful"; and that the claims about trafficking "cannot be substantiated."

Illusion

But by that time, the report had generated a mass of news stories, most of which took the unreliable results and overstated them. Like Chaste, the Poppy Project, which has been paid nearly £6m to shelter trafficked women, has been drafted in to advise police and until recently used office space in the Sheffield headquarters of the UK Human Trafficking Centre.

The cacophony of voices has created the illusion of confirmation.

Politicians and religious groups still repeat the media story that 40,000 prostitutes were trafficked into Germany for the 2006 world cup – long after leaked police documents revealed there was no truth at all in the tale. The Daily Mirror's baseless claim of 25,000 trafficking victims is still being quoted, recently, for example, by the Salvation Army in written evidence to the home affairs select committee, in which they added : "Other studies done by media have suggested much higher numbers."

Somewhere beneath all this, there is a reality. There have been real traffickers.

Since the Sexual Offences Act came into force in January 2004, internal police documents show that 46 men and women have been convicted and jailed for transporting willing sex workers and 59 people have been convicted for transporting women who were forced to work as prostitutes.

Ruth Breslin, research and development manager for Eaves, which runs the Poppy project, said: "I realise that the 25,000 figure, which is one that has been bandied about in the media, is one that doesn't really have much of an evidence base and may be slightly subject to media hype. There is an awful lot of confusion in the media and other places between trafficking (unwilling victims) and smuggling (willing passengers). People do get confused and they are two very different things."

She said that in the six and a half years since Poppy was founded, a total of 1,387 men and women had been referred to them, of whom they had taken in just over 500 women who they believed had been trafficked into sexual exploitation or domestic servitude by the use of coercion, deception or force. "I do think that there a lot more trafficked women out there than the women we see in our project. I do think there are significant numbers. I would say the figure is in the thousands. I don't know about the tens of thousands. That's probably going too far."

Certainly there have been real victims, some of whom have been compensated as victims of crime. The internal analysis of Pentameter Two, obtained by the Guardian, reveals that after six months of raids across the UK, 11 women were finally "made safe". This clashes with early police claims that Pentameter had rescued 351 victims. By the time that Brain held his press conference in July last year, that figure had been reduced to 167 victims who were said to have been "saved from lives of abuse, exploitation and misery".

However, the internal analysis shows that supposed victims variously absconded from police, went home voluntarily, declined support, were removed by the UK Borders Agency or were prosecuted for various offences.

Dealing with this, the document explains: "The number of 'potential victims' has been refined as more informed decisions have been made about whether or not the individual is believed to be a victim of human trafficking for sexual exploitation ... Initial considerations were made on limited information ... When interviewed, the potential victim may make it clear that they are not in fact a victim of trafficking and/or inquiries may make it clear that they are not and/or inquiries may show that initial consideration was based on false or incomplete information."

Research published recently by Dr Nick Mai of London Metropolitan University, concludes that, contrary to public perception, the majority of migrant sex workers have chosen prostitution as a source of "dignified living conditions and to increase their opportunities for a better future while dramatically improving the living conditions of their families in the country of origin". After detailed interviews with 100 migrant sex workers in the UK, Mai found: "For the majority, working in the sex industry was a way to avoid the exploitative working conditions they had met in their previous non-sexual jobs."

The UK Network of Sex Work Projects, whose outreach workers deal with thousands of prostitutes, told the home affairs select committee last year: "It is undoubtedly the case that women are trafficked into the sex industry. However, the proportion of sex workers of whom this is true is relatively small, both compared to the sex industry as a whole and to other industries." The chairman of that committee, Keith Vaz, observed: "We are told that this is the second largest problem facing the globe after drugs and we do not seem to be able to find the people responsible."

For the police, the misinformation has succeeded in diverting resources away from other victims. Specialist officers who deal with trafficking have told the Guardian that although they will continue to monitor all forms of trafficking, they are now shifting their priority away from the supposed thousands of sex slaves towards the movement within the UK of children who are being sexually abused. They say they are also dealing with more cases where illegal migrant workers of all kinds, including willing sex workers, find themselves being ripped off and overcharged for their transport.

Unheard

However, the key point is that on the sidelines of a debate which has been dominated by ideology, a chorus of alarm from the prostitutes themselves is singing out virtually unheard. In the cause of protecting "thousands" of victims of trafficking, Harriet Harman, the deputy Labour leader and minister for women and equality, has led the parliamentary campaign for a law to penalise men who pay for sex with women who are "controlled for gain" even if the men do so in genuine ignorance.

Repeatedly, prostitutes groups have argued that the proposal is as wrong as the trafficking estimates on which it is based, and that it will aggravate every form of jeopardy which they face in their work, whether by encouraging them to work alone in an attempt to show that they are free of control or by pressurising them to have sex without condoms to hold on to worried customers. Thus far, their voices remain largely ignored by news media and politicians who, once more, have been swept away on a tide of misinformation.

• This article was amended on Thursday 19 November 2009. We said that the Poppy Project had an office in the Sheffield headquarters of the UK Human Trafficking Centre. That is no longer the case. This has been corrected.