Harman's sex trafficking law is based on feeble, fraudulent evidence.

Here's the line. Women are being trafficked into Britain and forced to become sex slaves. We know this because the massive Operation Pentameter, involving 55 police forces, six government departments and various NGOs, led to the arrest of 528 sex traffickers. On the basis of this, Harriet Harman is rightly pushing through a bill to make it illegal to pay for sex with a prostitute controlled by someone else.

Except it's all lies. As Nick Davies reports, the six-month investigation actually failed to find a single sex trafficker. Ten of the 55 police forces arrested nobody at all. Some 122 of the 528 arrests claimed never happened (they were wrongly recorded, or phantom arrests designed to chase targets). Half (230) were women – suggesting that the Operation was a convenient excuse to harass prostitutes and clock up more arrest figures.

Of the 406 real arrests, 153 had been released weeks before the police announced their 'success', 106 without any charge at all, and 47 being cautioned for minor offences. Of the rest, 73 were charged with immigration breaches, 76 convicted on drugs raps, and others died or disappeared.

Only 22 people were finally prosecuted for trafficking, including two women. Seven were acquitted. The net haul from this vast operation was 15 successful prosecutions. Of those, just five men were convicted of importing women and forcing them to work as prostitutes (two of whom were already in custody).

So that's the 'huge success' that allowed Jacqui Smith and now Harriet Harman, to claim that 'thousands' of women were being trafficked, and to push a Bill through Parliament. So much for evidence-based policy: I would feel happier if they just said that they found prostitution disgusting and wanted to outlaw it for our own moral good. At least that would be honest. This is simple deception, a fraud on the public.

Sex workers are opposing the new legislation. They know that every time governments 'get tough' on prostitution, they are the ones who suffer. The police just have another excuse to go on fishing trips, round up a few girls, and boost their arrest figures so that they get Brownie points and the Chief Constable gets a better bonus. And to prove that they are not 'controlled', girls will start working alone, rather than in flats with a maid to look after them, which will make them more vulnerable to abuse and attack. Thanks, Harriet.

Dr Butler's book The Rotten State of Britain is now in paperback.