Thousands of people are being trafficked to the UK for forced labour in a "hidden crime" where victims go unnoticed, experts have said.
The workers are forced to put in long hours with little food while living in squalid conditions, the UK Human Trafficking Centre (UKHTC) said, and may be employed by firms unaware of the abuse.
UKHTC has launched a campaign with Crimestoppers and the Association of Chief Police Officers (ACPO) to highlight the plight of the workers.
James Behan, head of operations for UKHTC, said: "It's probably one of the least understood areas of human trafficking. If you asked a member of the public they would probably understand sexual exploitation and child trafficking, but when it comes to labour trafficking it's a hidden crime.
"The idea is to raise the awareness of the general public as they're going about their daily business to see the signs. It can range from a 15-year-old being made to work in a field to a middle-aged man who has fallen on hard times and become alcohol-dependent. It isn't one specific group, one nationality, one age group, it's very encompassing."
More than 1,000 victims of trafficking for forced labour have been referred to the centre since 2009 but Mr Behan said this could be "the tip of the iceberg", and he believes there are potentially "many more".
The workers are typically used in low-paid jobs where they have to work for long hours, have to live in poor-quality, cramped housing and can suffer malnutrition because they are fed so little. Jobs include being made to work in private houses as well as the hospitality, farming, manufacturing and construction industries.
Mr Behan said: "The people live in difficult conditions. Quite often they will share beds, it's very poor-quality housing and there's no real downtime because they have to work long hours. They have no possessions and no freedom."
British nationals have also fallen prey to forced labour. Recently, there have been high-profile cases where traveller families have been convicted of keeping men as modern-day slaves.
The Salvation Army said it wlecomed the Crimestoppers campaign. Major Anne Read, The Salvation Army Anti-Human Trafficking Response Coordinator said: "The impact of trafficking on the lives of those who are referred to The Salvation Army's care can be devastating, as shown by the story the young woman featured in the Crimestoppers campaign... We hope that this excellent campaign by Crimestoppers will be the means by which many more victims will be rescued from this all too prevalent evil. "