By June Kelly
BBC home affairs correspondent
Complex new legislation will take a lot of unpicking to prove, police say
New laws making it a criminal offence to have sex with prostitutes controlled by pimps may be too complex to work in practice, police have warned.
The legislation, which is due to come into effect later this year, aims to protect women forced into the trade.
Gloucestershire Chief Constable Dr Tim Brain said he feared the complexity of the law may make gaining evidence hard.
He said his concern was getting enough evidence "to merit a suitable number of prosecutions to act as a deterrent".
Dr Brain, the lead on prostitution for the Association of Chief Police Officers of England, Wales and Northern Ireland, said the legislation was "quite complex".
"It will take a lot of unpicking to prove and therefore I am concerned that the deterrent effect the government was hoping to bring about will be lessened because the legislation is so complex," he said.
The government's planned change to the law in England and Wales aims to protect women forced into the trade by traffickers and pimps.
Home Secretary Jacqui Smith launched the new legislation with an unequivocal message, saying "there will be no more excuses for those who pay for sex".
For the first time ministers have the "punters" in their sights.
At present it is not illegal to pay for sex. Under the new legislation a man will face prosecution if he pays for sex with a woman who is being "controlled for gain" by someone else.
And in court he will not be able to argue he did not know the woman was being controlled because ignorance is no defence.
Dr Brain said: "The idea that men should be responsible and have a wider knowledge of the harm they can cause by paying for sex in these circumstances is an absolutely sound principle.
"Our concern is around gaining sufficient evidence to merit a suitable number of prosecutions to act as a deterrent."
But what about those this legislation is aimed at - the men who have sex with prostitutes?
One man in his 60s called "John", who has been using prostitutes for most of his marriage, told the BBC the new law may deter some men but not him.
When asked if he would ask a woman if she was being controlled, he said: "I wouldn't expect her to tell the truth because it isn't a trade where the truth is always there.
"They're acting anonymously, I'm acting anonymously because there is something very wrong in what we are doing, so not telling the truth is only part of the story."
At a brothel in the south east of England with 50 women on its books, four women who were ready to see clients told the BBC they were all working in the sex trade voluntarily.
Andrea, who runs the business, said the new legislation would keep customers away and could force her to close and the women to work on their own.
"If the girls are working on their own they are open to danger," she said.
"They are one phonecall away from a rapist, a psychopath, muggers, anything... people know these women have cash on them. It's frightening."
While the women at Andrea's brothel say they are there from choice, there are many others forced to work in the sex trade who do not have a voice.
The Home Office says more than 160 victims of sex trafficking were rescued by police in a six-month period last year.
"We know there are many more women involved in prostitution who are being exploited in a number of ways," a spokesman said.
The bill also includes tougher controls on kerb crawlers.