The UK is seeing a rise in the number of rough sleepers being exploited and enslaved by criminal gangs, charities are warning, following the revelations that some of the "slaves" kept at a traveller camp in Bedfordshire were homeless people.

London-based homelessness charity Thames Reach told the Huffington Post UK that they had come across 20 cases in the last few months where people had come to them for help after fleeing captivity.

The warning comes after 24 people were found to have been allegedly held against their will and forced to work at a traveller site in Leighton Buzzard.

Mike Nicholas, of Thames Reach, said there had been "occasional rumours a year or so ago" about people being sold into slavery within the UK, but confirmation of the practice had come over the last few months including one case where a man was "bought" for £100.

Most of 20 the victims aided by Thames Reach are said to be of Central and Eastern-European origin. The charity said the eastwards expansion of the European Union in 2004 saw an increase in the number of people coming to Britain in search of work. While most are able to secure jobs, the charity said, some fall by the wayside and end up living on the streets. Unable to receive benefits and desperate for work they are prime targets for the capital's gangs.

Nicholas warned gangs were "preying upon desperate and vulnerable people" and keeping them in properties against their will. He said they were "turning up in vans and trying to recruit people" at homeless shelters and soup kitchens.

In one incident, men were forced to work 12 hours days and were paid only in alcohol, while the gangs kept the profits. Victims have also been physically abused. One man suffered burns on his wrists after being handcuffed to a radiator.

Victims have been put to work in, among other places, bakeries and mattress factories.

Paul Donohoe of Anti-Slavery International said the Leighton Buzzard case would not come as a shock to those working with homeless people on the ground.

So far four men have been charged with conspiracy to hold a person in servitude, requiring them to perform forced labour at the site in Leighton Buzzard.

Bedfordshire Police, who raided the camp on Sunday, said they found some of the alleged victims "living in their own urine".

Donohoe said the "harrowing" stories of the conditions the men were kept in has further exposed the problem of vulnerable homeless people being tricked into forced labour.

"What's really unusual is that you have first hand testimony of homeless people describing their conditions," he said.

But it has been reported that some of the men said to have been held against their will in Leighton Buzzard have returned to the site while others have refused to co-operate with the police investigation.

And Joseph Jones of the Gypsy Council questioned why, if they were being held against their will, they had not tried to escape previously.

"The idea you can keep people in prison without any gates beggars belief," he said. "The idea that people can't leave because they are too terrified of being beaten up ... who's going to beat them up if they get on bus and go somewhere else?"

But Thames Reach said often victims of forced labour will not try to escape captivity because they are "quite scared of retribution".

A spokesperson for the Serious Organised Crime Squad (Soca) said that they had helped Bedfordshire police out with specialist advice on how to help and interview vulnerable people extracted from these situations but was reluctant to comment further.

Statistics published buy antislavery.org provide some indication of the scale of the problem.

Using the national referral mechanism (NRM) - the official way for authorities to identify trafficked people – figures show that during the period 1 April 2009 to 31 March 2011 there were 1481 referrals to the NRM. Of those, 72 per cent were female, 28 per cent male. Surprisingly, 74 per cent were adults, compared to 26 per cent child referrals.

Of the 1481 referrals, 30.3 per cent were for labour exploitation, 18.4 per cent was for domestic servitude, leaving 43.6 per cent accounting for sexual exploitation.

Amnesty International declined to comment.