As such, migrants end up serving years in UK prisons despite being forced to commit the cultivation crimes by gangs who seize passports and threaten – and administer – violence.

“The abuses of freedom in cannabis farm cases do not tally with traditional perceptions of slavery. Victims may be held against their will, forced to work and unable to leave, despite an unlocked door,” said Prof Heather Strang, the study’s senior author.

“Big questions remain about how the criminal justice system should ethically manage modern slavery victims who are also illegal immigrants involved in illegal activity,” she said.

The new study, published in the Cambridge Journal of Evidence-Based Policing, was co-authored by DI Adam Ramiz of Surrey Police as part of his research at Cambridge, where he worked with Strang and Prof Paul Rock from LSE.

Cannabis farms are unassuming abodes in towns and city suburbs that house hundreds of plants in blacked-out rooms, grown with equipment such specialist lighting. A live-in “grower” will work for criminal gangs to feed and protect the Class B drug crop.

The latest study is small in scale – gaining access to growers willing to talk is difficult – but criminologists say that it’s an important addition to this under-researched area.

The team looked at criminal histories of 19 Vietnamese nationals arrested in connection with cannabis farming in Surrey and Sussex between 2014-2017, and conducted in-depth interviews with three further growers – two Vietnamese and an Albanian – as well as the arresting officers in those cases.

The growers all described being in hock to human smugglers, working in farms to pay debts, and some spoke of death threats and physical intimidation. Two spoke of dangerous journeys to the UK via lorries, similar to the 39 Vietnamese nationals found dead in Essex last year.

One witnessed murder by smugglers while trekking for days through forests. Another was locked inside the house once in the UK. The victims didn’t consider themselves such, as they had wanted to come here, yet had been forced into illegal labour on arrival: smuggling that becomes trafficking.

Interviews with officers revealed police questioning on slavery to be limited, cursory and “binary” – whether or not the grower was physically locked in – and conducted with a presumption of guilt on the that the grower is an offender.

“We found that some officers only had an hour of modern slavery training, and felt that the onus is on trafficking victims to volunteer that information, rather than police to investigate further,” said Ramiz, who led the study.

“The brief question or two on slavery will often come after a grower has been given the standard legal advice to say nothing and later to plead guilty,” he said.

Police frustrations focus on growers, with one officer talking of “hitting a brick wall” if they won’t open up, but researchers say that the legal advice offered to trafficked cannabis growers is routine and uncritical: “go quietly”.

They argue that police should “re-frame” their response to cannabis farms so that the possibility of modern slavery is “more fully considered”, and suggest detailed training for front-line officers along with greater willingness to refer cases to specialist investigators.

Dame Sara Thornton, the UK’s Independent Anti-Slavery Commissioner, described the study as a “welcome contribution to building an evidence-based approach to preventing modern slavery”.

“The Modern Slavery Act includes a statutory defence for those compelled to commit an offence as a direct result of their being a victim of modern slavery. It is essential that the police investigate all lines of enquiry when they come across these complicated cases,” said Thornton.

Added Ramiz: “While much more research is needed, these accounts of debt bondage and fierce intimidation suggests the mass cultivation of cannabis is rife with modern slavery, and the grey area between offender and victim in these cases can become a blind spot for UK police.”

Case Study:
A 34-year-old Vietnamese man now in an English prison for growing cannabis told researchers he had been a taxi driver, before fleeing his home after taking part in protests against a Chinese oil rig in the disputed South China sea. Accused of betraying his country by police, he entered into contract with a smuggler after fearing for his life when a friend disappeared following arrest. Unable to pay in full, he ended up in debt bondage to a criminal gang.

Believing he was going to the UK to work in kitchens, the grower found himself in a series of lorries and flights across China and Russia, and taken into Europe via the forests of Poland.

“You have to walk for maybe two, three days… I saw one person had been beaten up… when I turned around he was unconscious… he walked too slow,” the grower told researchers. He believed the person he described had died.

The grower arrived in the UK in a lorry container. He was eventually taken to a house already full of cannabis plants and shown how to tend them, and given an allowance for food and phone calls home. “I do not dare leave the house without telling them, because I fear for my life… They told me if I tried to escape they would harm my family,” said the grower.

He remembered police asking some questions about being forced to work, and he had told them. His legal advisor asked no such questions. He did not consider himself a trafficking victim, as he had wanted to come to the UK.

The police interviewer of the grower was a 33-year-old probationary police officer. He had been given an interview plan, and told researchers he viewed the matter in simple terms: “…you’re interviewing him as a suspect to get a confession, or to get the points across to get the conviction or charge…”.

No trafficking questions were in the officer’s plan, but he asked some anyway based on the grower’s response. The officer acknowledged his ignorance of modern slavery legislation to researchers.

A further interview was done by the officer’s supervisor, who was in charge of the investigation. He told researchers the training given to police on slavery – one hour-long session – was insufficient, and until guidance improved they had to rely on instinct.

The officer-in-charge entered a submission to the National Referral Mechanism – the framework set up in 2009 to ensure victims of trafficking receive help. The NRM returned a decision that the grower had “consented” to the illegal work, so was not a victim, and he was sentenced to prison.