"The growing body of evidence we are collecting points to the scale being far larger than anyone had previously thought".
Annie Reid, director for anti-trafficking and modern slavery for the Salvation Army told Premier that while the Christian charity isn't surprised by the NCA's report, things need to change to help vulnerable people.
"We know modern slavery and human trafficking a much bigger problem than is being identified, yet we're still stumbling around because we don't have proper data", she told The Independent.
Agency official Will Kerr says investigators have found widespread evidence of people as young as 12 being sold for sexual exploitation or forced labour.
Britain's National Crime Agency (NCA) revealed on Thursday it is now working on 300 live policing operations targeting modern slavery.
Others sold into slavery could be kept in pop-up brothels - where sex workers promised a better life are instead left penniless - while some work in cannabis factories.
A surge in operational activity focusing on labor and sexual exploitation coordinated by the NCA through May and June, codenamed Operation Aidant, led to 111 arrests in Britain and some 130 people being encountered who may be considered as victims.
"It is my hope that the campaign launched today by the NCA leads to an increased awareness among the British public, so that we can pride ourselves on being a nation that will not tolerate the evil of modern slavery".
Kerr said signs of slavery included visible injuries, a distressed appearance and any indication they are being controlled by another person.
Mr Kerr cited the case of a 12-year-old Roma girl stopped at border control, bound for a life as a domestic slave after she was "effectively sold by her father".
He said: "She was being brought in to work for a family in part of the United Kingdom, 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".
"There will be people living and working where victims come into contact with everyone else's so-called normal lives", said Mr. Kerr, who appealed for members of the public to be vigilant.
Police in Bristol arrested four people last week following a raid at a nail bar in Southmead, Bristol, following a tip off over concerns for a woman.
A 43-year-old woman was arrested at an address in Chesterton Court in Chester on suspicion of modern slavery.
A Home Office spokesman said the government had taken "world-leading action" to tackle modern slavery, including toughening up sentences and increasing support and protection for victims.
Three people have been arrested in Cheshire as part of an investigation into modern slavery and human trafficking offences.
Aidan McQuade, director of the UK-based Anti-Slavery International, said cuts to police numbers and a general lack of understanding among rank and file officers have been contributing factors in the rise.
His comments came as the NCA revealed the scale of the modern slavery problem in Britain.