Britain's Home Office (interior ministry) estimates there were up to 13,000 victims of slavery in Britain in 2013. Victims were most often from Albania, Nigeria, Vietnam and Romania.

In this video, Chloe Setter of ECPAT UK, a charity working with child victims of trafficking, describes the plight of Vietnamese children who are trafficked to Britain for cannabis cultivation.

"To my knowledge, there's never been a prosecution of a Vietnamese trafficking gang for bringing children in for this purpose," said Chloe Setter, Head of Advocacy, Policy and Campaigns at ECPAT UK, a charity that works with children who have been trafficked.

"We've actually locked up, prosecuted and convicted more victims unlawfully than we have prosecuted those that are exploiting them," she told the Thomson Reuters Foundation.

CLICK HERE to read the full story by Katie Nguyen at Reuters