Det Ch Insp Sean O'Neil said: "The men we found at the site were in a poor state of physical health and the conditions they were living in were shockingly filthy and cramped.

"We believe that some of them had been living and working there in a state of virtual slavery, some for just a few weeks and others for up to 15 years."

He added that the men appeared to have been "recruited" from soup kitchens and benefits offices and included people with problems such as alcoholism.

He said: "They're recruited and told if you come here we'll pay you £80 a day, we'll look after you give you board and lodgings.

"But when they get here, their hair is cut off them, they're kept in in some cases [in] horseboxes, dog kennels and old caravans, made to work for no money, given very, very small amounts of food.

"That's the worse case. Some are treated a little bit better, but they were told they could not leave and if they did they would be beaten up and attacked.

"But in fact some people did leave and told us what was going on and when we looked back since 2008 we were aware of 28 people who had made similar accusations."