Britain risks “losing the fight” against human trafficking unless the criminal justice system urgently improves its response to the crime, a major study will warn.

The report, seen by The Independent, comes at a time when the number of trafficking victims identified is soaring and criminal convictions for the offence have plummeted. It is published by the Anti-Trafficking Monitoring Group (ATMG), a coalition of organisations established to scrutinise the government’s progress in tackling modern slavery.

The group is calling for a unified law on the crime to make it easier to prosecute - something the Government is understood to be considering.

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It found “widespread” evidence that many trafficked people are prosecuted for crimes they are forced to commit, while the criminal bosses who enslave them go unpunished. Despite legal protection for victims, the ATMG says the practice of unlawfully punishing them is common.

One woman was imprisoned for 12 months for possession of a false passport and having documents used for fraud, despite Judge Guy Kearl QC stating, “I accept that you have been a victim of trafficking and you were exploited.” In another case of trafficking from Vietnam for cannabis farming, the defendant was sentenced to 24 months, despite the judge acknowledging he had been coerced.

Klara Skrivankova, trafficking programme co-ordinator at Anti-Slavery International, said: “Until the Government makes tackling trafficking a priority it won’t be effective in prosecuting traffickers and protecting the victims. Our evidence suggests that many trafficked persons are prosecuted for crimes they were compelled to commit while their traffickers enjoy impunity. This is unacceptable. The UK is obliged by the law to investigate traffickers and protect victims from criminalisation.”

The report says “countless” examples where the police “did not recognise the crime at all” were highlighted to ATMG charities. It says human trafficking is “not a policing priority,” despite the Government’s commitment to make Britain a hostile environment for traffickers.

In one recent case, four Hungarian men who had been trafficked for labour attempted to obtain out-of-hours police assistance at four different London police stations. In all of four they were met with indifference and told this was a civil matter and not a police concern. They were only helped after an intervention by the Metropolitan Police’s Trafficking and Prostitution Unit.

In another case, a man trafficked from South Asia attempted to report labour trafficking to the police, wishing to give details of the trafficker whom he claimed was exploiting others. He attended a police station in Manchester and another in Leeds and was refused help by both, who advised it was an immigration matter that should be dealt with by the Home Office.

Cases where trafficked children were re-trafficked on release from Young Offender Institutions were also identified by the group, which said it was concerned about a “revolving door” effect, leading to vulnerable children becoming victims twice.

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Chloe Setter, head of advocacy at ECPAT UK, said: “We consistently see child victims of trafficking not being given adequate protection and support. There continues to be a gap in the understanding of frontline practitioners who just aren’t well-enough informed about how to recognise and deal with cases of trafficking.”

She added: “Prosecutions of child traffickers are rare, leaving children without justice and their perpetrators free to re-offend. This sends out a dangerous message that the UK is not tough enough on child trafficking.”

The report highlighted many arbitrary decisions by the UK Border Agency, which is responsible for deciding if someone is trafficked or not. In one example, two West African girls were trafficked at the same time to the same brothel, yet one was recognised as trafficked while the other was not. This happened despite them both participating in a high profile and ultimately successful investigation against their trafficker.

The number of trafficking victims identified in Britain continues to rise. In September 2011, 36 victims were identified, while by February 2013, 83 were found in a month. At the same time, the number of traffickers successfully prosecuted is plummeting, with just 8 convictions in 2011, half as many as in 2010 and a third of those in 2008.

Many victims’ unawareness of their rights in Britain were exploited by traffickers. In one case reported to the ATMG, the trafficker dressed in a police uniform and raped the victim to reinforce fear of the authorities, and generate the belief of corruption within the UK police.

A Government spokesman said: “Human trafficking is abhorrent and we are committed to combating this crime in all its forms. We have already made significant progress in the fight against trafficking. Every prosecutor has legal guidance and access to training and thousands of front-line professionals, including border staff and police have been trained to better identify, support and protect vulnerable individuals.”