Picture: Jae C. Hong/AP/African News Agency (ANA) Archives

London - Traffickers are using drugs to force a growing number of British people - often homeless or mentally ill - to work for them, a charity supporting victims of modern slavery said on Wednesday.

The Salvation Army said it provided support services to almost 1 900 people - including Albanian and Nigerian women trafficked for sex work and Vietnamese forced to grow cannabis - in one year up to July, up 19 percent on the year before.

"One significant change we have seen ... is an increase in the number of British adults referred to us following forced criminality in dealing drugs," said Kathy Betteridge, the charity's anti-trafficking director.

"Traffickers have coerced people either using existing problems with substance addiction or controlled them through forced use of substances," she said in the report which showed that British survivors almost doubled to 86 in the year.

There are about 136 000 modern slaves in Britain, according to the Walk Free Foundation's 2018 Global Slavery Index - about 10 times more than a 2013 government estimate. Most are trapped in forced labour, sexual exploitation and forced marriages.

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 labour.

The Salvation Army said traffickers systematically targeted homeless people and those with mental health issues, or a history of drug or alcohol abuse.

"I was in a very difficult part of my life and being taken advantage of," a 34-year-old British man was quoted as saying in the report.

The homeless man, who had been an addict for many years, was referred to The Salvation Army after the police realised that he was not a criminal, but a victim of the drug dealers, working for them without pay.

Jakub Sobik, a spokesman for anti-trafficking group Anti-Slavery International said such cases showed the complexity of dealing with trafficking.

"Many people who are treated like a criminal are in fact vulnerable victims," he told the Thomson Reuters Foundation in emailed comments, adding that children were also being increasingly trafficked into the drug trade.

Thousands of children - some as young as 12 - are estimated to be used by gangs to carry drugs between cities and rural areas in Britain, according to the National Crime Agency (NCA).