Hien was 10 when he arrived in Britain. He did not know where he was or where he had been. He knew only that he was here to work. Since he emerged from the back of a lorry after crossing from Calais seven years ago, his experience has been one of exploitation and misery. He has been a domestic slave, been trafficked into cannabis factories, been abused and beaten and was eventually prosecuted and sent to prison. It has been a life of terror, isolation and pain.

Hien’s story is not unique. He is one of an estimated 3,000 Vietnamese children in forced labour in the UK, used for financial gain by criminal gangs running cannabis factories, nail bars, garment factories, brothels and private homes. Charged up to £25,000 for their passage to the UK, these children collectively owe their traffickers almost £75m.

While there is growing awareness of the use of trafficked Vietnamese people in the booming domestic cannabis trade, child trafficking experts are now warning that the British authorities are unable to keep up with the speed at which UK-based Vietnamese gangs are recruiting and exploiting children for use in other criminal enterprises such as gun-smuggling, crystal meth production and prostitution rings.

“By our calculations there are around 3,000 Vietnamese children in the UK who are being used for profit by criminal gangs,” says Philip Ishola, former head of the UK’s Counter Human Trafficking Bureau.

“The police and the authorities are now aware that trafficked children are being forced to work in cannabis farms but this is really only the tip of the iceberg. Often the same child will be exploited not just in a cannabis farm but also in myriad different ways. This is happening right under our noses and not enough is being done to stop it.”

Police admit that they are struggling with the speed at which Vietnamese criminal gangs are diversifying and expanding their activities across the England and into Scotland and Northern Ireland. “Right now we are just fighting in the trenches, fighting in the nail bars,” said detective inspector Steven Cartwright, who heads Police Scotland’s human trafficking unit. “It is vital that we that we understand new methods being deployed by the gangs because we need to stop demand at one end or limit their ability to make money at the other.”

Hien’s journey to the UK started when he was taken from his village at the age of five by someone who claimed to be his uncle. As an orphan, he had no option but to do as he was told. He spent five years travelling overland, completely unaware which countries he was going through, from Vietnam before being smuggled across the Channel and taken to a house in London. Here he spent the next three years trapped in domestic servitude, cooking and cleaning for groups of Vietnamese people who would come in and out of the property where he was held.

The men in the house beat him and forced him to drink alcohol until he was sick. Other things happened to him that he still cannot talk about. He was never allowed out of the house and was told that if he tried to escape, the police would arrest him and take him to prison.

During his time in that house, Hien says, many other Vietnamese children were brought in. They told him that they were here to work and to pay off debts for their families back home. They would stay for a few days and then be taken away, and Hien never saw them again. He became homeless after his “uncle” abandoned him. He slept in parks and ate out of bins. He was eventually picked up by a Vietnamese couple, who offered him a place to stay but then forced him to work in cannabis farms in flats in first Manchester and then Scotland.

In his testimony to police, he says he still does not understand exactly what the plants were, although he understands now that they are worth a lot of money. He looked after the plants, using pesticides that made him ill, and only left the flat when he helped transport the leaves to be dried elsewhere. He was locked in, threatened, beaten and completely isolated from the outside world.

“I was never paid any money for working there,” he says. “I did not stay there for money but because I was afraid and I hoped the whole thing would end soon.”

When the police came, they found Hien alone with the plants. He told his story to the police, but was still sent to young offenders’ institution in Scotland, where he spent 10 months on remand, charged with cannabis cultivation. He was released only after the intervention of a crown prosecutor led to him being identified as a victim of trafficking.

Vietnamese children such as Hien are easy pickings for the increasingly sophisticated trafficking gangs operating between the UK and Vietnam. Children make up nearly a quarter of the estimated 13,000 people trafficked into the UK every year, and Vietnamese children are the largest group of children trafficked to the UK. The United Nations Office on Drugs and Crime estimates that 30 Vietnamese children arrive illegally in the UK every month, on well-established smuggling routes.

“Children are an increasingly valuable assets to criminal gangs because they are easy to get hold of, easily intimidated and exploited, and easy to keep isolated and unaware of what is really happening around them, which makes it far less likely for them to be able to disclose anything of use to the police,” says Ishola.

When it comes to Vietnamese children, he says, the culture of seeing a child as the “golden egg”, who will be sent to work abroad and provide for their families still prevails. This attitude is exploited by gangs, who deceive families into believing that there is legitimate work in Britain for their children.

“During their journey to the UK, the traffickers keep charging the children more and more money, and by the time they arrive, the pressure to pay back this enormous debt is a key factor in their vulnerability to ending up trapped in forced labour,” he says. “Upon arrival the children are faced with a highly organised system of criminal activity, with methods of control ranging from extreme physical brutality to debt bondage. Before they even arrive, that trap is set for them.”

Members of the Vietnamese diaspora in London told the Observer that they had seen an explosion in child trafficking by criminal gangs operating on the peripheries of their communities in recent years. “Some of these children and victims have told me that it cost them £25,000 to get to the UK,” said one Vietnamese community leader in London, who did not want to be named. “They come with a debt and they are not allowed to leave until the debt is paid. That is slavery and exploitation.”

Hien’s story: A child trafficked from Vietnam into forced labour in the UK Guardian

Like Hien, many of the children end up working on cannabis farms. The link between child trafficking and the UK’s domestic cannabis industry has been increasing, with Vietnamese children the main group at risk. According to a 2014 report by the NGO AntiSlavery International, almost all potential victims of trafficking linked to cannabis are Vietnamese, and more than 80% are children. Many of these children are subsequently prosecuted by the UK justice system, despite many being identified as potential victims of trafficking. This has led to Vietnamese children becoming the second-largest ethnic group held in youth detention centres across the UK.

Vietnamese gangs have historically dominated the UK’s £1bn cannabis trade and have been instrumental in the proportion of domestically grown cannabis in Britain rising from 15% in 2005 to about 90% now. While the trade remains enormously profitable – the number of Vietnamese cannabis factories in the UK has grown by 150% in the past two years – their grip has been weakened thanks to increased law enforcement and under competition from British growers. Now they are finding new and more efficient ways of doing business.

“In terms of law enforcement, I think we’re about two years behind the curve,” says Daniel Silverstone, a criminologist at London Metropolitan University who has written extensively on Vietnamese gangs in the UK.

“Traffickers have changed their modus operandi in recent years in direct response to the attention and interventions of law enforcement. A few years ago it was almost exclusively cannabis farms, but their business interests have now become much more diverse. So we’re seeing an expansion into Scotland and Northern Ireland, the use of nail bars for forced labour and money laundering, and moves into drugs like crystal meth.” This means that children, who are an integral part of the gangs’ business operations, are also now being moved into other areas of exploitation. “As their grip on the domestic cannabis trade slips a little, they are looking to maximise their profits from these children in whatever way they can,” he adds.

The Metropolitan police say that there is now much more awareness of the complexity of tackling the UK’s child trafficking problem but that the closed nature of the Vietnamese community has made things difficult. “What has persistently been a challenge for us is making inroads into this community,” says Phil Brewer, who heads its new human trafficking and kidnap unit. “We usually only find out about a child when we make a raid and find someone in a cannabis factory or nail bar, but often this person has been through multiple forms of exploitation before we reach them.”

Parosha Chandran, a leading human rights barrister and UN expert on trafficking, has represented Vietnamese children charged with cannabis cultivation who have gone through many different trafficking situations before being moved into cannabis farms.

“Trafficked Vietnamese children have rarely faced just one type of forced labour,” she says. “I’ve come across cases where young people have been subjected to a spectrum of exploitative practices. In one of my cases, for example, the child was forced to look after people’s homes and care for their children, when he was just a child himself, then he was taken to work cleaning a nail bar, then moved to another place where he was forced to sew labels on to clothing – and all of this happened before he even arrived in the cannabis factory.”

In March the UK passed its first Modern Slavery Bill, designed to increase the prosecution of traffickers and give better protection to victims of modern slavery in the UK. However, Chandran says that Vietnamese children continue to be prosecuted for cannabis cultivation while their traffickers remain free.

“The Modern Slavery Act’s central focus on prosecution is misguided and its provisions fail to fully protect the rights of trafficked children,” she says. “We as a democratic country need to find durable solutions to ensure these children remain protected from harm for the rest of their lives.”

At safe accommodation for child trafficking victims run by the charity Love146, Lynne Chitty, its UK care director, says that she has helped between 40 and 50 Vietnamese children try to rebuild their lives after trafficking experiences.

“We have seen children starting to be exploited in multiple ways, to maximise the profit that can be gained from them,” she says. “We recently had a client who was in domestic servitude, forced to work in a nail bar during the day and every evening taken to a brothel and exploited there all night.”

Methods used to lure children from Vietnam to the UK are also becoming increasingly sophisticated, including use of social media. “Vietnamese children are brought to the UK, taken in by Vietnamese adults and put to domestic work,” says Swat Pandi, from the NSPCC’s child trafficking advice centre. “The child feels indebted to the adults for food and shelter and is told they need to return the favour by looking after cannabis plants. These children suffer high levels of neglect, emotional abuse and, in the absence of any protective factors, are highly vulnerable to physical and sexual abuse.”

Despite the government’s pledge to end modern slavery and the UK’s first modern slavery bill, passed in March, Chitty says she has seen no change in the numbers of Vietnamese children coming through her charity’s services. “It’s very much business as usual,” she says. “We still have a problem with immediate safeguarding and appropriate placements for trafficked children. And young people are still being criminalised by the courts.”

Even when a child has been taken out of trafficking and come under the care of a local authority, he or she is likely to return to the control of the traffickers. In 2013, a report by independent thinktank the Centre for Social Justice concluded that 60% of trafficked children in local authority care go missing, nearly a third of them within a week of arrival. Most are never found again. There are increasing reports of children being retrafficked from foster homes or when they have been given asylum status.

“I don’t think we understand the entire enterprise,” says detective inspector Cartwright. “Despite our best intentions I think we’re not offering them anything that would persuade them to stay. Many will get retrafficked because we didn’t offer them a better alternative to what the traffickers are providing.”

Hien is trying to rebuild his life after being given asylum in Scotland, but is struggling to find peace after years of trauma. “I still worry that the traffickers may find me and come to my house. But I know this time that I will ask for help,” he says. “I think they have justice here but I wish they hadn’t kept me in prison for so long. By telling my story, I want people to understand what I have experienced here.”

Additional reporting by Neil Loughlin and Kieran Jones

MODERN SLAVERY IN NUMBERS

10,000

The Home Office estimates that there are between 10,000 and 13,000 victims of modern-day slavery in the UK.

100

Estimates suggest that there could be 100 children trafficked every week into the UK. Albanian and Nigerian females, including adults, make up the largest groups being referred to agencies as potential victims, while Vietnamese people make up the largest number of males referred. British children make up the largest group of trafficked young people, while Vietnamese children are the largest number of foreign nationals exploited in the UK.

34%

The National Crime Agency has reported a 34% rise in potential trafficking victims in 2014 compared with a year earlier. Adults are predominantly victims of sexual exploitation, while minors are exploited for labour.

51%

The National Society for the Prevention of Cruelty to Children (NSPCC) says that 51% of all the young people referred to itschild trafficking advice centre from Vietnam have been reported missing at one time.

96%

Anti-Slavery International says that of the potential trafficking victims who were forced to cultivate cannabis, 96% were from Vietnam and 81% of those were children.

25%

Nearly 25% of all trafficking victims are children.

£30,000

Agencies report that victims are being sold on, along with their debt, for as much as £30,000, to other traffickers for multiple exploitation, including sex trafficking, domestic servitude and cannabis cultivation.

58%

Of the trafficked children who have disappeared, the NSPCC reported in its 2012 all-party parliamentary groupreport that 58% were being exploited for criminal activity and cannabis cultivation.