A Vietnamese man suspected to have been trafficked to the UK and forced to work in a cannabis farm has been detained for two years after the Home Office dismissed warnings that he had been exploited.

Ministers have been accused of “criminalising” modern slavery victims by “dismissing crucial evidence” after it emerged that the man in his twenties was held in detention despite medical evidence suggesting he was a victim of trafficking and exploitation.

It comes after the bodies of 39 people, all believed to have been Vietnamese nationals, were discovered in a lorry in Essex last month – an incident that has prompted concern that punitive immigration measures are pushing migrants into the hands of people smuggling gangs.

The tragedy has also raised questions about Britain’s approach to tackling modern slavery, with campaigners warning that victims often fear reporting the abuse due to fear of being detained and deported.

The man, who The Independent is not naming to protect his identity, arrived in the UK in 2015 hidden in a fruit and vegetable van along with 11 other Vietnamese nationals. Two years later, he was arrested after police found him working on a cannabis farm.

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He was referred to the National Referral Mechanism (NRM) – the UK’s framework for identifying trafficking victims – but was sentenced to 18 months for cannabis cultivation before any decision was made.

Almost a year later, the NRM concluded that on a balance of probabilities he was not a victim of trafficking, and he was detained under immigration powers.

While in detention, numerous medical reports by both doctors within the removal centre – known as Rule 35 reports – and external medical professionals raised concern that the man was a potential victim of torture, and highlighted a number of indicators that he had been exploited and abused.

A Rule 35 report from July 2018 stated: “He was attacked and beaten on many occasions in a house in England – he does not know where he was. He was kept and forced to work and would be attacked if he was not compliant. He was punched and kicked and had watery acid on his face and says he cannot see from his left eye. He was beaten with sticks also.

“He has nightmares and depression and feels an increase in his symptoms since being detained with flashbacks also. My opinion is that continued detention, with the uncertainty of his status and the restrictions of his freedom, will mean his mental health suffers further.”

A second Rule 35 report, from July 2019, stated that despite taking anti-depressant and anti-psychotic treatment for several months, there had been no improvement in the man’s mental state.

It added: “He is also experiencing nightmares related to past trauma and sleep difficulties and he could be developing delayed-onset symptoms of PTSD […] which cannot be satisfactorily treated in the detention setting. As a result his mental health is detrimentally affected by detention.”

The Home Office dismissed these reports. The man was released last week when his solicitor, Shalini Patel of Duncan Lewis, wrote to the department after it failed to respond to the most recent Rule 35 report, written by a senior clinical psychologist last month, which warned that he was now on suicide watch and was not fit for detention.

But despite the report recommending that he be put in touch with the mental health team prior to release, Ms Patel said this did not happen, and that he was instead released without any of his medication and no details as to how he could register with a GP.

And although he has been released from detention, she said the Home Office was “completely refusing” to consider whether a second referral into the NRM is justified, meaning he could be detained again.

Campaigners said the case demonstrated that the government was “criminalising” victims of trafficking and “failing to protect vulnerable individuals” despite clear concerns they are not fit for detention.

Ms Patel told The Independent: “This client was detained under immigration powers for a longer period of time than he spent in prison for a criminal conviction for which he was not guilty of. How someone can be accepted as a potential victim of trafficking but simultaneously also be convicted for a crime he was trafficked into is just abhorrent behaviour.

“The police want victims to report exploitation, but why would anyone come forward if this is how they are treated? It is about time the authorities used evidence to protect these vulnerable people and target the traffickers.”

Shadow home secretary Diane Abbott said the man’s treatment was “completely unacceptable” and a product of the government’s hostile environment policy, adding: “It undermines the entire fight against this evil trade, as victims will refuse to come forward.”

Emily Kenway, senior adviser at Focus on Labour Exploitation, said the case was “yet another egregious example of the Home Office’s continual prioritisation of administrative immigration issues over the rights and needs of victims” and accused ministers of “ignoring” trafficking indicators.

“There have been shameful failures on the part of government agencies at every stage in this man’s case. Clearly, [he] should never have been convicted while his modern slavery status was still being considered and questions need to be asked about how this was allowed to occur,” she said.

The case comes after a major report in July revealed that the government had knowingly held hundreds of modern slavery victims in removal centres in potential breach of the law, with detention used in some cases to supposedly safeguard vulnerable and traumatised individuals.

Ms Kenway accused the government of “clearly failing” to meet its responsibility under the Council of Europe convention on trafficking to assist victims in their recovery due to its “obsession with treating undocumented migrants as criminals”.

She added: “Sadly, this case is not anomalous; hundreds of victims of trafficking are being locked up in immigration centres in the UK today, a practice which must be stopped immediately.”

Home Office forced to defend refusal to disclose detention of hundreds of modern slavery victims

Pierre Makhlouf, assistant director at Bail for Immigration Detainees, said: “All too often we encounter highly vulnerable people who have been controlled and exploited by human traffickers, criminalised and imprisoned by the justice system for offences that they were forced to commit.

“The Home Office is fully aware of the scale of this unfolding tragedy but is prepared to cause significant harm to individuals in its pursuit of immigration enforcement.”

A Home Office spokesperson said: “Modern slavery is an abhorrent crime and one the Government is committed to stamping out. All Home Office staff working in the detention system are given training to ensure potential victims of trafficking and slavery can be identified and provided appropriate support.