Joan Smith: Damaged women are safer now, thanks to Jacqui Smith

But how is it going to be enforced? I've lost count of how many times I've heard that objection since Jacqui Smith announced proposals to criminalise men who buy sex from women who've been coerced into prostitution. Personally, I don't see the problem: under the new law, ignorance won't be a defence and the police have to establish only that a woman is under the control of pimps or traffickers. Then the client can be charged with a new offence of paying for sex with someone who is "controlled for another person's gain". If the man has "knowingly" had sex with a trafficked woman, he could be charged with rape – and about time too. (For pedants, let me point out that the new law will apply equally to men and women. In practice, the vast majority of people who buy sex are male.)

It is three years since I first argued that men who have sex with trafficked women are rapists, and I am delighted that we now have a Home Secretary who agrees. I wish she had gone further and made buying sex a criminal offence in all circumstances, but I don't think opponents of the proposed law realise how clever it is. Advocates of legalised prostitution were up in arms last week, claiming that trafficked women are a tiny minority and that most women who sell sex do so voluntarily. Ms Smith takes a different view and made it clear in interviews: "We need to send a message to men and to society in general that most women do not choose to be in prostitution, whereas the buyers have free choice."

Suppose for a moment, however, that Ms Smith's opponents are right. Say that the majority of women selling sex on the street are not dependent on hard drugs, even though the Home Office believes that almost all of them are problem users; say that police chiefs are exaggerating when they suggest there are between 6,000 and 18,000 trafficked women in this country. If selling sex is as voluntary as its defenders suggest, why are so many people furious about Ms Smith's announcement? Clients have nothing to fear unless they think they are likely to encounter women who have been encouraged on to the streets by drug dealers and pimps, or forced to work as sex slaves by traffickers. And that's the brilliance of the proposed law: as well as protecting vulnerable girls and women, it challenges the notion that there is such an entity as "fair trade" prostitution.

This is the argument of the sex trade lobby, who claim that most of the problems associated with prostitution are caused by illegality. For them, it's largely a health-and-safety question; the answer is always some form of legalised brothels. But legalisation doesn't stop violence and sexual abuse, such as demands for unprotected and for anal sex. And the fact that sex-trafficking goes on worldwide – it's the third largest illegal industry after drugs and guns – suggests a severe shortage of women going into prostitution of their own free will. A shocking insight into the scale of trafficking emerged during a debate at the Council of Europe, when a delegate said there were few young women left in some villages in Moldova; they'd all been lured into the west European sex trade. Some may suspect that they will be required to sell sex for a while, but few realise that they might have 20 or 30 "clients" in a single day.

For too long, the law has made a tragic error, punishing women whose lives have been wrecked by the sex trade and ignoring the punters. Three cheers for Ms Smith. I hope her proposals will scare off thousands of British men who might otherwise go on paying for sex with homeless girls, drug addicts and savagely beaten Lithuanians.

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