Modern slavery and human trafficking in the UK is much more prevalent than previously thought, with cases affecting "every large town and city in the country", the National Crime Agency (NCA) has said.
The body is currently assisting on 300 live police operations targeting modern slavery, with alleged victims as young as 12 being sold to families in the UK from Europe.
Will Kerr, NCA director of vulnerabilities, said: "The more that we look for modern slavery, the more we find evidence of the widespread abuse of the vulnerable.
"The growing body of evidence we are collecting points to the scale being far larger than anyone previously thought.
"This should not be acceptable in any way, shape or form."
Mr Kerr's comments come as the NCA launches an advertising campaign raising awareness of the signs of modern slavery in everyday life.
He said there were "lots of different outlets" for people trafficked into the UK to be working illegally and against their will, with many affecting people in everyday life.
Mr Kerr said examples included those working at car washes and in construction as well as in agriculture and food processing - often receiving very little pay and forced to put up with poor living conditions.
Others sold into slavery could be kept in pop-up brothels, where sex workers promised a better life are left penniless with few clothes other than underwear, while some can be working at cannabis factories, he added.
"As you go about your normal daily life and as you're engaged in a legitimate economy accessing goods and services, there is a growing and a good chance you will come across a victim who has been exploited in one of those different sectors," he said.
"That's why we are asking the public to try and recognise the signs and to report their concerns and suspicions to us."
He said signs of slavery included visible injuries, a distressed appearance and any indication they are being controlled by another person.
Anyone with information about possible slavery gangs is asked to contact the NCA on 08000 121 700.
Mr Kerr said the most common nationality of victims brought into the UK were people from Eastern Europe, Vietnam and Nigeria.
But he also cited one example of a 12-year-old Roma girl being stopped at border control, bound for a life as a domestic slave.
He said: "She was being brought in to work for a family in part of the UK, where she had effectively been sold by her father - or it had been facilitated by her father - and she was being brought in to take this family's children to school and pick them up every day, and clean the house in between. Twelve years old, same age as my youngest son."
Mr Kerr said criminal charges were pending against those involved in the case.
But he said it underlined the sheer scale of the problem authorities were facing, further complicated by the fact some people do not realise they are a victim of slavery.
He added: "People are being exploited on an hourly and daily basis.
"The full scale and extent of it, we don't know. But what we have found is that in every medium-to-large town and every city in the UK, we have found evidence of vulnerable people being exploited.
"We can't put a figure on it but we can say there are tens of thousands of victims across the UK."
Earlier this year, the NCA released figures which showed the number of suspected victims of slavery and human trafficking had more than doubled in three years.
There were 3,805 people reported as potential victims in 2016, an increase from 1,745 in 2013, according to NCA statistics.
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