Some of up to 13,000 victims of modern slavery in Britain are being “failed” by “chronic weakness” in police responses to their plight, the UK’s Anti-Slavery Commissioner has warned.

Kevin Hyland has used his first annual report to issue stinging criticism of a series of police failings which he says could be allowing organised crime groups behind slavery and people trafficking “to act with impunity.”

The report, presented to Parliament on Wednesday, stated that “chronic weaknesses in modern slavery crime reporting ”, plus “a lack of intelligence reporting and evidence-based operational action” meant “victims both present and future are being failed”.

In some police force areas, Mr Hyland added, slavery victims were just being “lost” in the system.

Mr Hyland also reported that some British companies were still engaging in little more than “box ticking” when claiming their supply chains were slavery-free, and raised fears that the migration crisis and refugee camps like the Calais Jungle were being exploited by traffickers to enslave the vulnerable.

His criticisms come weeks after Prime Minister Theresa May announced the creation of a new task force involving the police, MI5 and MI6, aimed at ridding the world of what she called “the evil” of modern slavery.

Professor Bernard Silverman, the Chief Scientific Advisor to the Home Office, has estimated that at any one time there may be between 10,000 and 13,000 potential victims of modern slavery in the UK.

They can be lured to the UK with promises of a better life, only to find themselves so heavily indebted to those who transport them that they effectively have to work for their traffickers for nothing.

Traffickers have been known to photograph women being gang raped in order to force them to work in the sex trade under the threat of having the pictures shown to conservative family and friends back in their home countries.

In what one law enforcement official called “blackmail nicely packaged as traditional ritual”, Nigerian children have been terrified into submission by “juju” ceremonies where a “witch doctor” takes hair or blood from the victim and says they will die or fall horribly ill if they fail to obey their traffickers.

Vulnerable UK residents – particularly the homeless and youngsters in care – can also find themselves being trafficked within Britain, either for forced labour, or, in so-called “grooming” cases, for sexual abuse.

Law enforcement agencies have found victims of modern slavery providing a vast range of goods and services bought by unsuspecting UK customers. They have been found on the fringes of the UK food production industry, working in pop-up car washes and even delivering pizzas. Children from Vietnam are being forced to work in atrocious conditions in illegal cannabis farms hidden in suburban houses.

The Modern Slavery Act 2015 introduced life sentences for anyone convicted of slavery or trafficking offences in an attempt to change the nature of what many offenders viewed as a “high profit, low risk crime”.

But in his annual report, Mr Hyland highlighted continuing “inadequacies” in modern slavery crime recording that “could allow organised crime groups to act with impunity.”

He said that while the UK’s modern slavery National Referral Mechanism (NRM) identified 3,146 potential victims in the financial year 2015-16, English and Welsh police forces recorded only 884 modern slavery crimes.

This, he said, meant that “at best” 28 per cent of referrals to the NRM in the past financial year had resulted in a modern slavery crime being recorded by police in England and Wales.

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Some of this shortfall will have been because the NRM acts as a filter, allowing authorities to gather further information and determine whether an incident is a genuine case of slavery, or – as sometimes happens – an attempt to ‘game’ the immigration system and falsely claim asylum.

Sometimes the NRM will record crimes that occurred outside the UK as, for example, victims of trafficking or slavery in Europe escape to the UK.

Mr Hyland’s comments, however, made clear he did not believe that such explanations can wholly account for the gap in the amount of NRM referrals and the much smaller number of crime reports.

Introducing a detailed critique of how police forces handled NRM referrals, he said: “Some UK police forces are taking a proactive approach to combating modern slavery. I am disappointed, however, that many instances of substandard modern slavery crime recording remain.

“When NRM referrals are not recorded as crimes, investigations are not launched and victims do not receive the justice and support that they need and deserve.”

Mr Hyland found that more than half, (21 out of 39), of the English and Welsh forces who responded were unable to say whether a NRM referral made by their force had resulted in a modern slavery crime record, or expressed significant difficulties in doing so.

“Furthermore,” he added, “Four forces could not find any internal record of the NRM referrals made by their force (nor any crime record). For two such forces this was even after seeking assistance from the National Crime Agency’s Modern Slavery Human Trafficking Unit. Consequently, in a small number of forces, victims are apparently being lost.”

Mr Hyland also said that the National Crime Agency (NCA) – Britain’s equivalent of the FBI and the agency responsible for collating NRM data nationwide – had been “somewhat passive” in ensuring that reports of suspected slavery made by non-police bodies like charities were considered by the relevant local forces.

“Opportunities have also been lost to mine NRM data for the benefit of intelligence and investigative opportunities,” he added. “It is clear that the collection, recording and analysis of modern slavery data is substandard. From the limited data that is gathered on victims’ circumstances via the current NRM system through to compromised crime recording, a lack of intelligence reporting and evidence-based operational action, victims both present and future are being failed.

“It is paramount that precise modern slavery crime recording is a policing priority in 2017.”

Acknowledging Mr Hyland's criticisms, a National Crime Agency spokesman told The Independent: "‘We have noted the comments. Until relatively recently modern slavery was not identified as one of the NCA’s highest priority threats. This has changed, and we are currently working with the Home Office and law enforcement partners on a range of proposals to deliver a step change in this increasingly important area.”

Mr Hyland, who was appointed by Theresa May in 2014 when she was Home Secretary, also expressed concern that the migration crisis and camps like the Calais Jungle were exposing children and vulnerable adults to possible exploitation.

Having visited the Jungle, he said he had written to Home Secretary Amber Rudd “to express concerns about the current situation” and to recommend “proactive identification and resettlement of unaccompanied minors, where it is in their best interest.”

Despite years of scandals about goods sold in the UK being the product of exploited labour abroad, the Anti-Slavery Commissioner also said some British companies were still engaging in mere ‘box ticking’ when claiming their supply chains were untainted by slavery.

Mr Hyland said: “There is still much more to be done to ensure that companies produce statements that both comply with the [Modern Slavery] Act’s obligations and point to decisive action being taken, as opposed to merely being a ‘tick box’ exercise.”