The UK’s independent anti-slavery commissioner, Kevin Hyland, has criticised police forces for failing to tackle the issue of enslaved Vietnamese teenagers being trafficked to the UK to work in illegal cannabis farms.

There was a lack of urgency and an absence of commitment from police in dealing with this crime, he said, describing the intelligence-gathering system designed to tackle this and other forms of modern slavery as “a mess”.

Cannabis farms are discovered every week, he said, but when they are identified they are “not being properly investigated”. Police forces had “not pushed forward with the urgency I would expect,” he said in an interview. Despite the fact that Vietnam is consistently one of the biggest source countries for trafficked slave labour into the UK, there has never been a successful prosecution of a people trafficker from Vietnam, Hyland said.

Over the past decade police have become increasingly aware of a trafficking route from Vietnam that brings hundreds of young, vulnerable Vietnamese men and women to the UK every year. While women and girls are sent to work in nail bars or in the sex industry, young men and boys are forced to become gardeners in small, hidden cannabis farms, often set up in suburban houses.

Teenage Vietnamese cannabis farmers interviewed by the Guardian described being locked into terraced houses and left alone for weeks on end to tend to cannabis seedlings, with food delivered sporadically. They revealed the violence of traffickers and the gangs involved in drug cultivation, and the failure of British officials to realise that they were victims of trafficking. Many still end up in prison.

David Cameron raised the issue when he visited Vietnam in 2015, but British investigators have still failed to crack down on the trafficking networks. “It is very disappointing. We want to see more victims supported and identified, but also I want to see more people pursued and prosecuted,” Hyland said.

Usually when cannabis farms are raided the people who live there, tending the plants, are arrested. When police recognise that they are victims of trafficking, prosecutions are dropped, but often they are referred instead to immigration detention centres because they are here illegally. Those gardeners who are underage are put into the care system but frequently go missing and return to their traffickers.

Hyland said there was insufficient intelligence gathering done by police when they raid cannabis farms. “There is an easy opportunity for a prosecution of the person who is the cultivator,” he said, adding that few resources were spent searching for people higher up the chain. When a victim of trafficking is identified by police they are registered on the national referral mechanism, a National Crime Agency register designed to ensure they get the appropriate support. But Hyland said police had failed to use it as an investigative tool. “That rich bank of intelligence from the people who have suffered is being marginalised and almost being treated as irrelevant,” he said. “In simple terms, it is a mess.”

French authorities have had little success in closing down a holding camp, set up by traffickers in a disused coal mine in northern France, where Vietnamese children and adults wait to be smuggled into the UK, according to Mimi Vu, of the Vietnamese anti-trafficking organisation Pacific Links. She visited the camp in December and saw about 100 people sleeping rough, most of them in their mid-20s or younger.

“The site is very under the radar. They were huddled around oil drums, with fires to keep warm. There were some younger ones, about 15 years, who I felt very worried about,” she said. She said she is hoping to launch an education programme in Vietnamhighlighting the risks of traveling to the UK.

Cannabis cultivated with Vietnamese slave labour accounts for a significant proportion of the illegal crop grown in the UK, though the majority of those arrested are white British men. The latest National Police Chiefs’ Council report into cannabis noted: “Reports suggest a new trend of cultivation sites being controlled by white British organised crime groups, which employ Vietnamese nationals who are forced to work in cultivation.”

Hyland said there needed to be greater awareness of the conditions in which much of UK’s cannabis supplies are grown. “Most people who are using cannabis might know that it is funding some sort of criminality, but do they know the suffering at the end of that? People need to think very carefully that while it may be seen as a recreational drug, behind it could be wholesale abuse of young people and children who have got no freedom and no choice,” he said.

Chloe Setter, head of policy at the children’s anti-trafficking charity ECPAT UK, said: “It is hard to convey how traumatic life is for children exploited in growing cannabis. Essentially, it steals their childhoods and can have a devastating impact on their futures. Children are moved across the world and hidden in unsafe properties across the UK by organised criminal gangs. The situation is only exacerbated by the response from UK authorities who frequently fail to identify them as victims.”

• If you suspect someone to be a victim of modern slavery in need of assistance, or consider yourself a victim, you can call the Salvation Army’s 24-hour confidential referral helpline on 0300 303 8151.