LONDON (Thomson Reuters Foundation) - Large numbers of child slaves may be working on cannabis farms in London, experts warned on Monday, after new figures emerged showing the scale of cultivation of the illegal drug.

Police have found 314 illegal cannabis farms in the British capital since 2016, according to official data obtained by the London Evening Standard newspaper, the equivalent of one every two days.

Experts say children are being trafficked from Vietnam and other countries to work in these farms, which are often located in residential properties, and that the scale of the problem has been vastly underestimated.

“The high number of cannabis farms across London and trafficking of Vietnamese children to work in them is extremely worrying,” said Jakub Sobik, a spokesman for Anti-Slavery International.

“Potentially thousands of children and young people are being trafficked from Vietnam and exploited by ruthless criminal gangs,” he said.

Catherine Baker, policy officer at anti-child trafficking organization ECPAT UK, said authorities often treated child slavery victims as criminals rather than giving them the help they needed.

“It is vital that these children are seen by police as victims first and foremost and given proper support, as too often they are treated as criminals instead,” she told the Thomson Reuters Foundation.

London’s Metropolitan Police did not immediately respond to a request for comment.

Britain is considered an international leader in the fight against slavery having passed the 2015 Modern Slavery Act to jail traffickers for life, better protect vulnerable people, and compel large businesses to address the threat of forced labor.

Yet child victims of slavery have no guarantee of specialist support and campaigners say the law has not made a serious dent in a trade estimated to cost Britain billions of pounds a year.

In February, the British government was criticized for refusing asylum to a Vietnamese orphan trafficked into the country as a child to work in the cannabis industry.

Britain is home to at least 136,000 modern-day slaves, Australian human rights group Walk Free said last month - a figure about 10 times higher than a 2013 government estimate.

More than 2,000 suspected child trafficking victims were referred to British authorities last year, most of them trapped in sexual exploitation, domestic servitude or forced labor.

That was the highest number on record and a 66 percent increase on the previous year.

Hundreds were trafficked from countries such as Vietnam, Sudan, Eritrea, Afghanistan and Iraq, according to government figures.

“These vulnerable children are exploited in extremely dangerous conditions, with little or no pay and may be physically and psychologically abused by their traffickers,” said Baker.