It was still dark on the morning of 25 October 2013, when police smashed down the door of a seemingly empty two-floor house in a rural corner of Chesterfield. Once inside they found that their tip-off had been correct: the house, which at one time would have been a comfortable family home, was now a fully-functioning cannabis farm, complete with dozens of fully-grown cannabis plants, thousands of pounds’ worth of lights and equipment, and one terrified Vietnamese boy.

The boy had been asleep on a mattress in the living room when the police raid started. He had been jolted awake to the sound of loud banging and splintering wood as the front door gave way. The house, so long devoid of air and natural light, was suddenly flooded with flashlights and the noise of shouts and stamping boots. Minh (not his real name) scrambled backwards into a corner as he was surrounded by men in uniform asking him questions in English that he couldn’t understand. “I was very, very scared of these men,” he recalled recently. “But then I let myself believe that maybe they had come to rescue me.”

Enslaved on a British cannabis farm: ‘The plants were more valuable than my life’ – podcast Read more

Minh was one of the hundreds of children trafficked from Vietnam every year and forced to work in hidden cannabis farms across the UK: small cogs in the vast criminal machine that supplies Britain’s £2.6bn cannabis black market. Children such as Minh are valuable assets for those who run cannabis farms: cheap, expendable, and easy to control and intimidate. They are smuggled overland from Vietnam to the UK, and trapped in a form of modern slavery that is now widespread across Britain.

Official estimates say around 13,000 individuals are trapped in some form of enslavement across the UK, and Vietnamese people make up the third-largest group of victims, with more than half of them under 18. Over the last three years, the British government has identified 491 Vietnamese minors as potential child-trafficking victims, the majority of them teenage boys working in cannabis cultivation. These are only the ones who have been found. Thousands more are estimated to be working undetected in makeshift cannabis farms in suburban houses, empty flats, deserted warehouses and derelict industrial estates. Others are forced to work in nail bars, brothels and restaurants, or kept in domestic servitude behind the doors of private residences.

Minh was 16 when he arrived in the UK. When he emerged from the back of a lorry somewhere near Dover in June 2013, he had no idea where he was or where he had been since he left Vietnam. He only knew he was here to work.

His memories of the three months he spent locked in the house are fractured and distorted by the fear, loneliness and stress that consumed him. His only visitors were Vietnamese men who would appear at the house every few weeks to check he was looking after the plants properly. They barely talked to him, leaving boxes of frozen meat that he heated up in an old microwave in the kitchen. They always locked the door behind them when they left. Apart from that, he was always alone. Behind the blackout blinds, days merged into night and back to day. Inside, Minh sat in the dark and the filth. He was hungry all the time, and terrified his food would run out. After a few days, the sweet, thick stench of the cannabis buds was overpowering, making him sick with headaches and nausea. He knew he would be in terrible trouble if the plants died, so every day he carried buckets of water upstairs to the plants and mixed chemicals into the soil.

Once, he says, he tried to get away, but was caught and brought back to the house, and was made to understand that he’d be killed if he tried to escape again. “It was like another kind of world,” he says. “I didn’t really even feel human. I understood very quickly that the plants were more valuable than my life.”

The day of the police raid marked the end of Minh’s enslavement and his liberation from his traffickers. But his ordeal was not over. Instead, Minh would find himself trapped in a system that treated him as a criminal rather than a victim. After being enslaved by drug gangs, he would be illegally incarcerated and violently assaulted while under the protection of the state. His fight for justice would lead to a landmark high court battle that raised painful and troubling questions about how the UK treats foreign children who have been trafficked and enslaved on British soil.

Minh was born in poverty in a small village in the south of Vietnam in the mid-1990s. His mother and father were smallholding farmers who grew just about enough rice to feed the three of them. By the time he was 16, Minh longed to get away. When his chance came to follow some friends to Ho Chi Min city, he took it. He has not seen his parents since.

In Ho Chi Min, Minh says, he met up with friends and kicked around the city for a few days. They took him to a house full of much older men who he didn’t know. The men told him that they knew he was poor and needed work, and asked if he would he like to go to the UK. It was no problem, they said, that Minh didn’t have any money. He could pay them back when he started working.

Minh didn’t trust these men. He told them he wanted to go home. He looked to his friends for help, but they wouldn’t meet his eye. He says he was then dragged into another room, punched and kicked to the ground, and beaten with sticks. For the next few days he was kept in the house and forced to give the men oral sex. He was repeatedly raped. By the time the men showed Minh a piece of paper that said he owed them £20,000 for his passage to Europe, he was so terrified that he signed it. For extra insurance, the men told him they knew where his parents lived. If he didn’t pay the money back, they would hurt them, too.

Facebook Twitter Pinterest A police raid of a cannabis factory in a house in east London last month. Photograph: Lewis Whyld/PA

So began Minh’s journey towards the cannabis farm in Chesterfield. The route he was taken on – overland across Russia, through eastern Europe to France and crossing to the UK in the back of a lorry – is the same that hundreds, if not thousands, of Vietnamese children take every year. Many of those making this journey have chosen to leave home, paying smugglers up to £30,000 for their passage to the UK and the promise of work in the thriving Vietnamese diaspora when they arrive.

Child-protection experts say the distinction between illegal migration and trafficking is a fragile one. “I’ve interviewed about 40 Vietnamese children who have travelled this route, and all of them without exception have been harmed and exploited on the way,” says Mimi Vu, an anti-trafficking consultant. “There is this very deep-rooted belief that it is their filial responsibility to provide for their family, and that the debt is their burden to shoulder. It gives people who are looking to exploit and profit from them huge leverage.”

On his journey to the UK, Minh was passed from gang to gang, sleeping in a network of filthy apartments often crowded with other young Vietnamese boys and girls. He says he was beaten, starved and sexually assaulted. When Minh finally arrived in the UK, he knew no one, spoke no English, and he was frightened and in debt.

“These are just the prime conditions for exploitation,” says Vu. “The only people these kids know and can rely on by this point are their traffickers, who are in complete control of their lives.” Within hours of arriving, many of these children disappear into an underground world of illegal work.

The set-up that police found in Chesterfield – an empty property crudely converted into a cannabis farm using high-end but easily available growing technology, and one or two foreign “gardeners” – has been seen over and over again in cannabis busts nationwide. In 2014, the last year for which figures are available, police seized a total of 276,676 cannabis plants at thousands of properties across the UK, with an estimated street value of £62m.

“Vietnamese kids have always been trafficked into illegal work, but cannabis is a perfect industry for their exploitation,” says Vu. “It’s easy to conceal someone in an empty house, the police see cannabis cultivation as a low priority, and if they do raid the house, the kid is usually too terrified to share anything worthwhile with them. And they’ve broken the law, so they’re likely to be seen as criminals first and victims second.”

When a confused and frightened Minh was interviewed at Chesterfield police station in October 2013, he didn’t tell officers about how he had come to be in the cannabis house, or that he had been forced to look after the plants. “They didn’t ask, so I didn’t say anything,” he says. “I didn’t know I was allowed.”

Because he was 16, he was remanded into local authority care and told he would have to appear in front of a judge to answer criminal charges. There were no foster places available, so he was driven to a B&B on the outskirts of Chesterfield by a social worker and told to wait for someone to tell him what to do next. But every time Minh thought about the police or going in front of a judge, he felt like his heart would explode with panic. The only people he knew in the UK – his traffickers – had told him that if the police found him they would put him in prison and never let him out again. They said that if he got the chance, he should run. So that’s what he did.

He left the B&B with £30 that the social worker had given him and took a bus to Sheffield. “When I got off the bus, I was in a strange city,” he said. “Then I began to feel guilty about getting into trouble, so I tried to get back to the house, but by then I didn’t know the way back.”

Minh wandered aimlessly around Sheffield for two days, sitting in parks, picking food out of bins and sleeping at the train station. Then, on the third day, while sitting on a park bench, he was approached by an older Vietnamese man who spoke kindly to him in his own language and offered him food and somewhere to stay. Minh remained with the man and his family, first in Sheffield and then in Liverpool, for more than two years. But this all changed in February 2016 when he was picked up during an immigration raid in Liverpool. When his records were checked and his outstanding cannabis cultivation charge was found, he was arrested again.

This time, Minh told the police what had happened to him in the cannabis house in Chesterfield. They called the Home Office, who sent an immigration official to interview him. They immediately passed his details on to the National Referral Mechanism (NRM), which identifies and offers protection to victims of human trafficking. A few days later, the Home Office said there were reasonable grounds to assume Minh had been a victim of slavery.

At this point, his criminal charges should have been suspended, but the Crown prosecution service was never informed of the Home Office’s decision. Minh’s lawyer had no experience of trafficking cases and advised him to plead guilty. A month later, Minh’s case went to court and he was convicted of cannabis production and sentenced to eight months at Glen Parva young offenders institution in Leicestershire.

Get the Guardian’s award-winning long reads sent direct to you every Saturday morning

Being locked up again in Glen Parva had a catastrophic effect on Minh. Before it was closed in 2017, Glen Parva was notorious as one of the UK’s worst places to be incarcerated. A few years before Minh arrived, conditions had been condemned by the government’s Independent Monitoring Board (IMB) as “deplorable” and a “toxic melting pot” of gang rivalry, bullying and mismanagement. It was also, according to the IMB, dangerous, unsanitary and under-resourced.

Minh, who at the time still spoke no English, was locked in his cell for up to 21 hours a day. He says he was bullied by both inmates and staff, denied food and racially abused. There were a number of other Vietnamese teenagers at Glen Parva, most of them on cannabis cultivation charges, but they were of little comfort to each other. “We were kept apart, and even when we saw each other, what did we have to say? All of us knew that nobody was coming to help us,” says Minh.

Four months into his eight-month sentence, Minh – who was acknowledged as a model prisoner by staff – was told he was about to be released. “I thought I was going back to Liverpool,” he says. But two days before he was to be released, Minh was told the Home Office had decided to detain him indefinitely under immigration powers.

When he did leave, it was in the back of another prison van. In June 2016, he was taken from his cell in handcuffs, and would be held in a series of immigration detention centres for another 13 months.

Under official Home Office guidelines, detention is deemed unsuitable for trafficking victims because it is recognised as a potentially replicating the isolation, submission and physical constriction that they suffered at the hands of their traffickers. Despite this, the first piece of research into this issue, which was based on responses to extensive freedom of information requests and published this month, estimated that at least 507 victims of trafficking were in detention in prisons and immigration centres across the country in 2018.

Now, six years after Minh was found at the cannabis farm in Chesterfield, Derbyshire police have acknowledged that serious errors were made that led to his being detained as a criminal. When the officers who conducted the raid found Minh, they should have followed official protocol and used a set of human-trafficking indicators to identify that he was a potential victim.

Over the phone from Chesterfield, Det Insp Carl Chetwyn told me that Derbyshire police’s approach and awareness of modern slavery has changed since 2013, especially after the establishment of the UK’s Modern Slavery Act of 2015. “I really believe that what happened to that individual in 2013 wouldn’t happen now,” said Chetwyn. Derbyshire police now have one of the UK’s only dedicated modern slavery and trafficking units in their regional headquarters. “In a case like that, any child found inside a cannabis farm would and should be automatically taken out of that situation and given contact with social care and referred into the NRM,” said Chetwyn. “We can’t arrest our way out of this criminality.”

But across the UK, children found in cannabis farms are still being criminalised. “Training is so patchy that you have areas of good practice, but there are large parts of the UK where the police still don’t know how to recognise child-trafficking,” says Lynne Chitty, who is programme director for anti-trafficking charity Love 146 and has run a safe house for child-trafficking victims in the south of England for 10 years. “Kids who should be protected are still being badly failed.”

In March 2017, Kate Macpherson, a trainee lawyer with law firm Duncan Lewis, arrived at Brook House, a sprawling, low-rise immigration removal centre near Gatwick airport. Macpherson was there to assess a new client who had just been taken on by a senior colleague and had been identified as needing urgent assistance.

Macpherson made her way through the security checks and met up with a translator, and together they walked to a small interview room at the back of the centre. Through the glass Macpherson saw a small, slight Vietnamese teenager sitting bent over at a table dressed in a T-shirt and tracksuit bottoms. It was Minh.

Macpherson had interviewed other detainees in immigration detention before, but immediately saw that Minh was in extreme distress. He was pale and shaking, his body covered in a red raw rash. “He could barely communicate,” she says. “Even speaking Vietnamese to the translator, he was unable to answer very basic questions. His mind was all over the place. He couldn’t make eye contact. He had completely withdrawn into himself.”

“When I walked away from that first meeting I had this sense of real panic,” she says. “It felt really very wrong to be leaving this very young and extremely vulnerable young person in an environment that was so clearly causing him immense pain and psychological stress. I understood enough from what he’d told me that he was quite possibly a victim of trafficking.”

The next day, Macpherson’s colleague at Duncan Lewis, Ahmed Aydeed, started to go through Minh’s case notes. At 32, Aydeed is ambitious about using public law to force change in the UK’s immigration system. He had already taken on more than a dozen clients who had been trafficked in the UK, but Minh’s story stood out.

Aydeed and Macpherson met with Minh multiple times over the next month, piecing together what had happened to him since he arrived in the UK. Even though Minh was slowly revealing his story, something about his case bothered Aydeed and Macpherson. “We just got the sense that something really terrible had happened to him in immigration detention that he wasn’t disclosing,” says Aydeed. It took them a few more weeks to gain Minh’s trust before he could tell them what had happened.

Facebook Twitter Pinterest Morton Hall Immigration Removal Centre in Lincolnshire. Photograph: Heathcliff O'Malley/Rex/Shutterstock

Minh eventually described how, in October 2016, a few months after being transferred to Morton Hall immigration removal centre, he was attacked and sexually assaulted by another inmate, who trapped him in his cell and tried to rape him. Later that day, Minh and a friend saw his attacker in the dining hall and a scuffle broke out. When he was interviewed by staff about the commotion, Minh told them about the assault. Under their own policies, Morton Hall should have immediately launched an investigation and reported the attack to the police. But they did nothing.

For Minh, the sexual assault did serious damage to his already fragile mental health. It was, he says, like a kind of death, triggering vivid and violent memories of being raped by his traffickers. “It just felt like my life was over. I just understood that I was not safe anywhere,” he says. “I was very scared of the other inmates and that something like this would happen again. I knew I couldn’t trust the staff there to protect me.”

Aydeed experienced what he described as “cold fury” when he learned what had happened to his client in Morton Hall. “The attack had happened more than five months before we first met Minh, but nothing had been done to protect him,” he says.

When Minh’s lawyers demanded an explanation from Morton Hall, the management initially said they hadn’t considered the attack a serious incident, and that Minh “did not suffer as a result”. Yet Minh had repeatedly told medical staff he was a victim of sexual and physical violence. He also told them he was experiencing relentless flashbacks. He often couldn’t sleep, and when he did, he had violent nightmares about being hunted.

“What they had on their hands was someone who had repeatedly disclosed that he had experienced rape, trauma, abduction, trafficking and torture,” says Aydeed. “Their failure to take this assault seriously was at best incompetence and at worst an indication of the culture of disbelief and disregard for the detainees in their care.”

The solicitors threatened legal action if the Home Office didn’t reconsider Minh’s case. On 12 May 2017, the Home Office formally identified Minh as a victim of modern slavery, yet still refused to release him from detention. When the solicitors demanded the Home Office open an investigation into his trafficking, the Home Office launched proceedings to deport him back to Vietnam, setting a removal date of 26 May. Minh was hours away from deportation when his solicitors got the removal orders against him quashed.

“They just fought and fought us on his case,” says Aydeed. “Even when they had admitted that he had probably been illegally detained, even when they admitted that he was a victim of trafficking, they just would not release him. It was almost perverse.”

After Minh was identified as a victim of trafficking, Duncan Lewis commissioned an independent medical assessment. Dr Frank Arnold of Forrest Medico-Legal Services, a global specialist who helps document the experiences of victims of torture, visited Minh at Brook House. Afterwards, Arnold wrote a blistering report concluding that Minh’s physical scars were consistent with a history of violent physical and sexual abuse, and that he was suffering from acute PTSD. He had, Arnold’s report concluded, suffered considerably at the hands of both his traffickers and the UK government.

“During my time as anti-slavery commissioner, I saw child victims from Vietnam go through absolute hell,” said Kevin Hyland, who worked at the Metropolitan police before becoming the UK’s first anti-slavery commissioner in 2015, when the Modern Slavery Act was established. “And then when they get pulled into our criminal justice system, it’s as if they’re entering this vast machine, where once you’re inside it is almost impossible to change course. The Home Office ends up defending a position almost as if it’s gone beyond the point where they can see people as human beings,” he says.

Other anti-trafficking experts have warned that many trafficking victims, who often have insecure immigration status, fell prey to the “hostile environment” approach towards immigration adopted during Theresa May’s time as home secretary.

On the morning of 14 June 2017, Minh finally walked out of immigration detention and into a safe house arranged by the Salvation Army. It had been three years and eight months since he was escorted out of the cannabis house, imagining he had been rescued.

Meeting Minh today, it’s hard to square the slight, softly spoken young man in his early 20s, in pressed blue jeans and trainers and tidy, gel-spiked hair, with the horrors that he has lived through. Yet often in our interviews his face slips, and you see the boy trapped among the plants in that airless, empty house six years ago.

He says that after he was released from detention in 2017, he struggled to adapt. There were other young Vietnamese men in the safe house, but he sat in his room in a state of high anxiety thinking that someone was going to come and take him back to the detention centre.

Rachel Thomas, a clinical psychologist specialising in complex mental health and trauma, was hired by Duncan Lewis to write a psychological assessment of Minh after he was released. Thomas has done similar assessments of scores of other trafficking victims, but vividly remembers her meetings with Minh at his safe house. She recalls him looking “small and slight and extremely young, much younger than his actual age, and very fragile”.

In her report, she writes: “His sleep is currently limited to about four hours a night. He is woken by nightmares about being attacked and chased. He is kept awake by intrusive memories of his experiences at the hands of his traffickers. They have become more intense and frequent since his detention, particuarly following the sexual abuse … during his stay at Morton Hall.”

During his assessment, Minh disclosed something he had also told Dr Frank Arnold: that after his failed escape from the cannabis house in Chesterfield, his traffickers had held him down, made a cut on his genitals and told him they had inserted a tracking device into his body. Thomas said that even if there was no tracking device, the psychological impact on the 16-year-old Minh had been immense: “Even when he was in a safe place and had been recognised as a victim, he continued to believe that the traffickers were in ultimate control over his life,” she says.

After Minh’s release from detention, Aydeed and the team at Duncan Lewis launched legal proceedings against both the Home Office and the Ministry of Justice for failing to protect Minh, and for failing to launch an investigation after the attempted rape at Morton Hall. Over the next 18 months, the team also pursued a separate judicial review against the Home Office. They argued that Minh was a victim of child trafficking, who had been systematically failed by the Home Office, which had criminalised and illegally detained their client over a four-year period, and then attempted to deport him back to Vietnam before a criminal investigation into his human trafficking had been concluded.

For Minh, this battle for justice has been crucial in his own slow journey to some kind of recovery. “After the incident at Morton Hall it was like I had split into two different people,” he says. “I don’t know who I am any more, but maybe there is a way to build a life for myself again. I used to talk a lot and laugh, and wanted to meet people and see things, but now I don’t feel like that any more. When I look in the mirror, I see someone else there, someone I don’t recognise, who is much older and who has been through awful things.”

Sometimes, he says, he finds it strange to walk the streets and be surrounded by people who have no idea what he has survived. “Since I left detention I have always felt scared, especially when I thought about how I was trying to fight the police and the Home Office,” he says. “It would be easier to just leave it alone. But I have to try my best to get justice, to take back my life. I have to trust that things can be better.”

In June 2018, Minh won his judicial review against the Home Office. The government accepts that it illegally detained him in prison, and then under immigration powers in Glen Parva and in immigration detention. When Minh heard the news, he felt the pressure lift for the first time in months. “When they admitted that what had happened to me was wrong and I had been treated unjustly, I think I really understood for the first time none of this was my fault.”

Then, on a cold morning in November last year, Minh received an ecstatic message from the team at Duncan Lewis. A court of appeal had quashed his conviction for cannabis cultivation. He was no longer a criminal in the eyes of the UK authorities. For Minh, this felt like a rebirth. Ever since he had been released from immigration detention, he had been trying to relearn how to live in the world, just going about the normal business of cooking food and washing clothes and getting through the day. “But all the time, the fact that I was a criminal was there, it felt like a black mark on my forehead,” he says. “Now I have a bit of peace, now I can learn how to be a good person in this society, and maybe be of help, be a useful person.” Last month, Minh won £85,000 in compensation from the Home Office and the Ministry of Justice for the illegal detention and failure to protect him in immigration detention.

When asked to comment on his case, a government spokesperson said: “The welfare of vulnerable detainees is of the utmost importance, and we are carefully considering the implications of this case. We have made significant improvements in recent years, but … we are committed to going further and faster in exploring alternatives to detention, increasing transparency around it and improving the support available for vulnerable detainees.”

The final battle to get Minh leave to remain in the UK will be concluded in the next few months. He is convinced that if he is sent back to Vietnam there is a high chance he will be trafficked again, or killed by the same gang who forced him into debt bondage as a teenager. It is far from certain that Minh will be allowed to stay. Despite talking itself up as a global leader in the fight against modern slavery, the Home Office only grants 12% of trafficking victims leave to remain. A recent investigation by Buzzfeed UK found that the government approved only 16 out of 326 applications made by children officially recognised as victims of slavery who had requested discretionary leave to remain between April 2017 and the end of 2018.

Inside the 21st-century British criminal underworld Read more

But Aydeed remains positive, and Minh wants to stay and try and build a life for himself in the UK, despite everything that has happened to him here. For the first time, he is allowing himself to think about what he would like to do with the rest of his life. He is only 22.

“All I can do is try to hope,” he says. “My life didn’t begin well, but that doesn’t mean that it has to be like this for me any more.”

If you have come across someone who may be a victim of slavery, call the Salvation Army’s confidential referral helpline on 0300 303 8151

• Follow the Long Read on Twitter at @gdnlongread, and sign up to the long read weekly email here.