In January 2018, a terrified 18-year-old Vietnamese man walked into a police station in London and told police that he had spent five years being trafficked in and out of cannabis houses by criminal gangs across the capital.

In halting English, he tried to explain how he had been taken from Vietnam, travelling overland across Europe, before being put in the back of a refrigerated lorry in France and brought to the UK to work in cannabis cultivation, he says.

Yet instead of interviewing him as a potential victim of modern slavery, the police called the Home Office, his lawyers say. He was then detained under immigration powers and taken to Brook House immigration removal centre where he remained until lawyers from Duncan Lewis managed to get him released.

The man, known as KQT in legal documents, has now launched legal action in the high court against the Metropolitan police for their failure in the legal duty of care to potential victims under the 2015 Modern Slavery Act.

The Met police said that it would be defending the claim, adding that it would therefore be inappropriate to comment further. But Ahmed Aydeed, the lawyer from Duncan Lewis who is representing KQT in his civil compensation claim, says his client’s case is one of many where the police have treated those reporting modern slavery as criminals instead of victims.

Facebook Twitter Pinterest Brook House immigration removal centre in Sussex. Photograph: Gareth Fuller/PA

“The police are asking victims to come forward and report their exploitation,” Aydeed said. “But what this case, and many other cases we have worked on, shows is that the police’s policy of reporting undocumented migrants to the Home Office helps traffickers and smugglers remain immune from prosecution.”

After 39 people were found dead in a refrigerated trailer in Essex last week, it quickly became clear that people like KQT could be of crucial importance to the investigation. The police initially indicated that the victims were Chinese, but as it emerged that families in Vietnam were missing sons and daughters who had made the long journey to Britain, the focus of the investigation changed.

That led investigators to ask other Vietnamese people who are in the country illegally to “take that leap of faith” and make contact with the authorities. But, warns Aydeed, the experience of people like KQT makes them unlikely to do so.

“Victims are not going to report their trafficking or assist the police if this is what happens to them when they attempt to get help,” says Aydeed. “They’re told by traffickers that the police will refer them to the Home Office and in many cases this is true.”

On Friday last week, DCI Pippa Mills was at pains to emphasise that “no criminal action will be taken” against anyone living illegally in the UK who came forward with information that could help the investigation. She also said that any tips would be treated “in the strictest confidence”.

On Wednesday Essex Police refused to elaborate on those remarks. They would still leave the way clear for the force to refer anyone to the Home Office whose information did not refer specifically to the case in Essex – and experts warn that in any case many migrants have no faith in official assurances.

Facebook Twitter Pinterest DCI Pippa Mills asks people to come forward with information. Many migrants have no faith in assurances no criminal action will be taken if they help the police. Photograph: Yui Mok/PA

Under new guidance issued in 2018, it is police policy to share information with the Home Office if they believe a victim of crime could be an illegal immigrant.

“This shows a central conflict in the police’s approach to fighting modern slavery,” says Aydeed.

Despite the 2015 Modern Slavery Act stipulating that anyone found in modern slavery should be considered a victim and not a criminal, trafficking victims can end up being detained under immigration powers, with one piece of research this year finding more than 500 were in detention facilities in 2018.

Those who are not detained often face the threat of being sent back to Vietnam, despite research that shows how dangerous this is, with many victims swiftly retrafficked back to the UK by the same gangs.

Three of the Vietnamese men discovered in 2017 locked up inside a cold war-era nuclear bunker in Wiltshire, tending 4,600 plants in 20 subterranean rooms filled with cannabis plants, were arrested and subsequently deported for immigration offences, even though police said there was “no doubt” that they had been enslaved. None of them spoke English, and they had been sleeping on mattresses on the floor. Modern slavery charges were dropped against the people running the drug gang.

In another case, Nam (not his real name) who was brought from Hanoi to the UK aged 12 and rescued during a police raid when he was 16, was later told by the Home Office that he had no valid asylum case and must return to Vietnam. His traffickers had brought him across the Channel in the back of a freezer lorry, and told him he owed them a debt of $100,000 for the journey to the UK.

Nam, now 21, had been identified as a trafficking victim and placed in foster care, so he was horrified to get a letter two years ago telling him he would have to go back to Vietnam. “I was worried the traffickers would take me again and bring me back here and I would have to risk my life again.” The Home Office’s decision to remove him was only reversed last year after a protracted legal battle.

Nam said he was having nightmares about the deaths. “I feel like I am in that lorry as well. I traveled the same way; I can feel how tragic they felt before they died. I can’t stop thinking about it.”

Facebook Twitter Pinterest A young Vietnamese man who was trafficked to the UK to work in a cannabis farm. Photograph: David Levene/The Guardian

Over the past decade there has been increasing work done to raise awareness among law enforcement and first responders of the inextricable links between modern slavery and the illegal migration of Vietnamese nationals into the UK. Vietnamese nationals are consistently among the top three nationalities to be identified as victims of trafficking, according to figures by the National Crime Agency.

Yet despite this, Phil Brewer, the former head of the Met’s anti-trafficking and kidnap unit, says little progress has been made in understanding the highly organised and ruthlessly efficient business model operating both the smuggling rings into the UK and the exploitation that often follows.

“The big issue for police on this issue is that whoever is responsible for bringing people into the country illegally can just make them disappear once they enter the country,” he says. “With the Vietnamese criminal networks in particular they are so well-established and so tightly run and with a sophisticated system of debt bondage, coercion and control in place that those they are exploiting are utterly dependent on them.”

Social workers have highlighted for years the problem of trafficked Vietnamese children, discovered at the ports, disappearing from foster homes within days to return to traffickers, aware that their families at home will face reprisals if they do not continue to work for them. Brewer says the way the system operates currently means there is “no incentive” for anyone to come forward to report.

Debbie Beadle, of the anti-trafficking organisation Ecpat, said the Home Office’s readiness to send people back to Vietnam was one of the main obstacles to persuading victims to come forward. “They think it is better to stay underground, and continue to work for their traffickers.”

Those working on the frontline with Vietnamese slavery victims say that many have been traumatised by the deaths of last week. Lucy Leon, of the Children’s Society, who has worked with hundreds of trafficked Vietnamese young people, said the journey to the UK was usually the most traumatic part of their experience. “They travel in overcrowded lorries, witnessing violence, and are petrified. The news has been hugely distressing.”

Philip Ishola, executive director of Love146, a UK anti-trafficking charity, said the impact of the news had “been terrible to watch”. “They all know this could have been them or someone they know. All have suffered at the hands of traffickers in the UK. There is a lack of trust and contact with the authorities and until we create a situation where people know they would be protected if they come forward this isn’t going to change.”

Alongside the horror at last week’s tragedy, there is a trace of optimism among some campaigners that this could be the tipping point that finally forces governments to tackle these trafficking networks.

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Jakub Sobik of the Anti-Slavery International charity said he hoped it might finally focus public attention on an issue campaigners have been warning about for years, just as the drowning of the Kurdish three-year-old Aylan Kurdi forced attention on the migrant crisis of 2015. “We hope this may hit the message home.” He called on the Home Office to treat victims with greater sensitivity. “Victims are still siding with traffickers more than the authorities because they are scared of officials, and rightly because there is more and more evidence of people being sent to jail for involvement in cannabis production, regardless of the fact that they have been locked in.”

Mimi Vu, a leading anti-trafficking campaigner based in Hanoi, said news of the deaths had shocked Vietnam, where most people had little understanding of the risks taken by people who travel to the UK.

“People have grown up thinking life in the UK is rainbows and popsicles. What is good about this awful situation is that people now know the reality. This should help when we do outreach and try to tell people you won’t be earning the amount of money they say you will. The traffickers lie about the dangers and about the money,” she said. But until there are better job opportunities in Vietnam, people would remain vulnerable to the promises of traffickers, she warned.

“Once this dies down, people will still travel to the UK because they still have no options,” she said. “Now they just know it is dangerous.”