There has been a 300% increase in the number of victims of modern slavery referred for support in the past six years, and a huge increase in the number of men from Vietnam trafficked to work in illegal cannabis farms.

Almost half (48%) of those referred to the government programme were brought to the UK for sexual exploitation, 39% were trafficked for labour exploitation and 13% for domestic servitude, according to figures released by the Salvation Army.

The charity, which is contracted by the government to deliver all support services for adult victims of slavery in England and Wales, also revealed that one person was trafficked for organ removal.

The rise in the number of referrals, from 378 in 2011 to 1,554 last year, is a reflection both of the growth in numbers of modern slaves in the UK, but also a reflection of improved identification of victims, the charity said. The service also reported an increase in the number of people trafficked from China, India and Pakistan.

The highest number of victims overall came from Albania, with 359 referrals, 346 of whom were women trafficked for sexual exploitation. But the most striking rise in referrals was the increase in the number of men from Vietnam. Last year, the Salvation Army helped 101 men from Vietnam, double the number it registered in 2015.

The charity’s annual report highlights the case of one young man whose mother sold the family home for £10,000 so he could travel to the UK, aged 16, to join his father who had left to find work, because he was unable to make ends meet as a farmer in rural Vietnam. He failed to find his father and was instead forced by traffickers to farm cannabis. He was locked alone inside a flat with blacked-out windows, not paid, and had food brought only occasionally by a man who would drop it inside the building and lock the door again.

His traffickers told him that he owned them money for his transport and lodgings, and that the debt had grown to £100,000. He spoke no English, and was not confident enough to go to the police, so he continued to grow cannabis and was also forced to work as a prostitute. It was only when police raided one of the farms where he was working and he was sent to a detention centre that he was identified as a victim of trafficking and referred to the Salvation Army.

“The fact that we are seeing such as increase in male victims from Vietnam, who have been forced under threat to themselves and their families, to farm cannabis, could reflect an improved ability to recognise the victims of these crimes,” Anne Read, the charity’s director of anti-trafficking and modern slavery, said.

“Sadly cannabis farming is one of the most easily hidden forms of modern slavery and such an increase in victims being referred to The Salvation Army for support could also indicate the scale of the problem and that we are still only seeing the tip of the iceberg of people being exploited in this way.”

The UK’s independent anti-slavery commissioner also recently highlighted concerns about the numbers of men and women from Vietnam being trafficked into the UK. The issue of Vietnamese workers cultivating cannabis farms has also increasingly appeared in the news, with a number of high profile raids this year, including the discovery of a nuclear bunker in Wiltshire converted into a vast cannabis factory, tended to by young Vietnamese men.

The figures also showed a rise in the number of victims from Poland, with 89 Polish men and nine women referred to the service for support. The report documented the case of one man in his early 20s who was promised work in the UK by a group of Polish men, who offered to organise his travel.

“When he finally arrived in the UK, K found that he was being forced to work in a number of different places, including car washes,” it said. “He was moved from place to place, his working conditions were poor and he was never paid for the work that he did. When K spoke to his traffickers about how badly he was being treated, they threatened violence not only to him but also to his family, claiming that they knew his home address in Poland and that they would go here and hurt his relatives. They also showed him a collection of weapons that they said they would use on anyone who went against them.”

Victims who are referred to the Salvation Army receive supported accommodation for a set period, legal advice, and information about whether they can stay in the UK and apply for work, or whether they must return to their home country.