"Buy Sex - Pay the Price," scream the posters in Brixton Tube. A silhouette of a shady man stands underneath the headline, moodily checking his mobile phone. "Lambeth Council and Lambeth police are targeting those who buy sex," explains the blurb, helpfully reminding commuters that "Prostitution is a form of violence against women and girls." Similar posters and billboards have suddenly sprung up all over the borough.

Those unaware of the complex and contradictory prostitution laws in Britain could be forgiven for thinking that prostitution is illegal. It isn't. And nor is buying sex. Whatever your views on the inherent "violence" in the practice, there's no getting away from this very simple fact. Although it's now possible to prosecute punters using the services of coerced or "controlled" women - a law that's been notoriously difficult to put into practice - only acts of public soliciting, such as kerb crawling, are a criminal offence. "If you choose to buy sex there will be consequences," states the campaign. Except there won't be.

Britain's prostitution laws date back to the 1950s when the Wolfenden Report decided that, despite the "immorality" of the practice, this alone was insufficient to make it illegal. Instead, prostitution was adjudged a matter of "public nuisance" - similar to drunkenness and Simon Cowell - and a framework was established to keep it out of sight. This framework included criminalising brothels, which means that, over 50 years later, it is still perfectly legal to work alone as a prostitute from the highly risky setting of an otherwise unoccupied flat but completely illegal as soon anyone else is present on the premises to, say, intervene in a rape. These fusty and damaging laws remain far more interested in keeping Britain tidy than in protecting vulnerable women.

On the surface, the new campaign by the Lambeth Partnership might seem like a good thing. At least, it is argued, that by targeting (or frightening) the johns rather than the sex workers, you're setting your sights on the less vulnerable party. Other countries have actual laws (rather than scare campaigns) to tackle this. In Sweden, for example, it is actually illegal to buy sex. But, it's widely argued, this simply drives the practice underground. Prostitution is a risky business. Nor is it feasible to eradicate it. Selling sex stills takes place in Sweden. It's just less visible - and thus less easy to police.

When I worked on a documentary about these issues a couple of years ago, we meet several sex workers who objected to being told what they could and couldn't do with their bodies (generally by men). They also argued, persuasively, that it was their regular clients who tended to look out for them. One thing we can all agree on is that the safety of vulnerable women is paramount. But by criminalizing punters, you remove the first line of defence. No-one is going to report apparent abuse at a "house of ill repute" if attending one in the first place implicates you in a crime.

According to Niki Adams, a spokeswoman for the English Collective of Prostitutes, the Lambeth campaign is another example of the sort of "heavy-handed moralism" that's been on the increase over the past few years, culminating in a desire to keep Britain - and especially London - looking green and pleasant in time for the Olympics. Measures such as these make it "more dangerous for sex workers" and are "aimed at increasing stigma and shame around prostitution." For Adams, it is areas such as Soho that are the safest places for sex workers because they remain "relatively visible and integrated as a part of the community." But this isn't necessary what Britain wants to show its Chinese tourists.

Meanwhile, as Lambeth pretends to crack down on punters and the spending cuts take hold, Adams has noticed a significant increase in people working in the industry. Calls to her advice centre from prostitutes have doubled over the past year. "It's mostly mothers, students and members of the immigrant community," she says.

For Lambeth Met Police Superintendent Andy Howe, "all [street workers] have fallen into prostitution as a last resort." This may well be true, but one wonders how targeting their clients is going to help them find a more palatable career. One also wonders how the campaign's promise that "people who use the services of prostitutes will be indentified, arrested and prosecuted" can possibly be put into effect. The whole thing smacks more of a toothless morality campaign than a sustained strategy to protect drug addicts, rape victims and women in need of urgent help. While Lambeth does provide some excellent support services for these women, "tough enforcement actions" are not part of the solution. Under the existing legal framework, nor are they even viable.