According to the police and the homeless charities modern day slavery cases are on the rise and becoming more common with 37 reported cases this year so far where people were forced to work for no pay.
Thames Reach, which works with homeless people in London, said that so far this year, it was aware of at least 37 incidents involving vulnerable people who had been forced to work for little or no pay and even made to break the law, compared with 22 last year.
Megan Stewart, reconnections manager with Thames Reach, said after the Bedfordshire conviction of four people who were controlling and exploiting homeless people were getting better at spotting the signs and even the police were taking it more seriously when being reported.
The exploitation involves trafficking people into the UK but also targeting homeless people on the streets.
The Passage Day Centre in London's Victoria, which helps homeless people, said its clients were regularly targeted both at the centre and at soup runs.
Clarke said the gangs were benefiting from the economic situation where economic downturn meant people were ready for being exploited.
In many cases, those who were exploited had been offered alcohol as "a reward" and had been told that they or their families back home would face violence if they reported what had happened to them.
A man from eastern Europe who was referred to Thames Reach by St Thomas's hospital had been trafficked into the country by a gang. When he complained about not being paid, he was beaten up and left on the streets with brain damage.
Much of the exploitation appears organised. Thames Reach said it was aware of reports that a criminal gang was trafficking people from the Lithuanian town of Panevezys on a twice-weekly basis.
There was a wide range of exploitation, said Detective Inspector Kevin Hyland, operational head of the Metropolitan police's anti-trafficking unit, which now operates four joint investigation teams focused on Bulgaria, Romania, the Czech Republic and Poland.
Hyland said his teams had uncovered examples of men who were trafficked into the UK to work in illegal casinos and who were required to give sexual services to gamblers during their breaks.
The Department for Communities and Local Government has worked with the Passage to fund a campaign in Poland to highlight the issue. Embassies in eastern Europe and the Middle East are also helping to raise awareness of the risk involved in working in the UK.
Hyland said vulnerable people often did not recognise their predicament. Maybe because of their mental state or because they are dependent on drugs or alcohol, they may not be able to realise they were being exploited, he added.