Few convictions under law for using prostitutes
A law intended to criminalise men who use prostitutes has led to just 43 convictions in England and Wales in its first year of operation.
Figures released to the BBC show that in some regions not one man was convicted for having sex with a woman who had been forced into prostitution.
Police say the law is difficult to enforce as it relies on women coming forward to give evidence of coercion.
Supporters say the legislation will take time to become fully effective.
The law, officially known as Section 53A of the 2003 Sexual Offences Act, is meant to reduce the number of trafficked women who work as prostitutes.
It allows police to prosecute men who have sex with women even if they did not know the woman had been forced to work as a prostitute.
Greater Manchester Deputy Chief Constable Simon Byrne, who is the Association of Chief Police Officers' lead on the issue of prostitution, said he was "surprised" at how many convictions there had been because the law "is difficult to prosecute".
Speaking to the PM programme on BBC Radio 4, Mr Byrne said: "The whole law in relation to this particular part of policing is confusing. We are calling for a simplification.
"We are looking at a range of options in dialogue with the Home Office to try and simplify things and to look at good models of practice in other parts of the world."
Acpo claimed last year that at least 2,600 prostitutes working in brothels in England and Wales had been trafficked from abroad, almost one in 10 of the estimated 30,000 working prostitutes.
But those figures are disputed by people working in the sex industry. They argue that most women are engaging in consensual sex, simply to earn money.
Niki Adams, from the English Collective of Prostitutes, says the law does not address the fundamental issues.
"I don't think this law should be used at all," she told the BBC. "It undermines sex workers' safety and it targets the wrong people. It targets clients who may be involved in consenting sex rather than the rapists and traffickers who should be targeted by the police."
The overall aim of the legislation was to dissuade men from using prostitutes, as men can be convicted even if they did not know the woman was coerced into selling her body.
The law was championed by many women in the last Labour government, including the former Solicitor General, Vera Baird.
She disputes that the 43 convictions show the law is proving ineffectual - but says it does need support.
"It needs to be promoted and the government has to do that," she said Ms Baird.
"They need to make very clear to these men just what they are sustaining. Prostitution is part of organised crime, these are not just isolated women - a huge proportion of them are run by pimps."
Despite the police's call for clarity, the government told the BBC that it has no plans to change the law on prostitution.
A Home Office spokeswoman said: "Police are well equipped to use Section 53 legislation and we want to see them do so."