Children are being used in criminal operations on Britain's streets in what the anti-slavery commissioner has called "Oliver Twist scenarios".
Youngsters who should be at school are tasked with pickpocketing, shoplifting and begging, Kevin Hyland has warned.
The former Metropolitan Police detective was appointed in November to spearhead the Government's fight against modern slavery.
In an interview with the Press Association he revealed that he had found "inconsistency" in how authorities approach the issue around the country and described the number of convictions for slavery offences as "nowhere near good enough".
'Oliver Twist scenarios' are happening on Britain's streets
Slavery can involve sexual exploitation, forced labour, domestic servitude or forced criminality, he said.
He said: "We hear about young children being sent out to do shoplifting, pickpocketing and begging. These are young children who should be at school.
"We are in the 21st century and yet we have Oliver Twist scenarios happening in our streets and cities. I think we all thought that was gone and in the past but actually it's alive and kicking in London.
"It's pure criminality and abuse of children. But it's all very complicated because of the way the children are groomed."
Modern slavery is a "very lucrative" business, he said.
"People are making a lot of money. In sexual exploitation, someone can make a million pound a year out of 10 women. We see cases of forced labour where people are making hundreds of thousands a month," Mr Hyland said.
Last year an inquiry found 1,400 children were abused by gangs of men, mainly of Pakistani origin, in South Yorkshire between 1997 and 2013.
Reports now suggest that authorities in Rotherham were warned about child sexual exploitation in 2003 but failed to act.
In December the Home Office published figures estimating there are between 10,000 and 13,000 potential victims of slavery in the UK.
They include women forced into prostitution, domestic staff and workers in fields, factories and on fishing boats.
Mr Hyland said official intelligence numbers were much lower than the Government estimate.
He said: "It's a mixture of things, it's not being reported, it's being missed, even when it's being reported is it being wrongly reported?
"I see cases where I'm meeting victims and hear their cases have not been investigated properly. These are things that really need to change."
He said he accepted the Home Office figure as an "accurate evaluation on the facts as they are", describing it as "a horrendous number".
"You have to remember these people are a victim of these crimes every day. It's not like a one-off incident, it's day in day out."
Kevin Hyland, anti-slavery commissioner, said Modern slavery is a 'very lucrative' business
There were 151 convictions for slavery related offences last year, Mr Hyland said: "It's nowhere near good enough.
What's really worrying is the numbers of investigations aren't sufficiently high.
"The reason why people are choosing this form of criminality is because there aren't the resources tackling it.
"To a degree they are operating in a low-risk area with high profit. A lot of my role has to be to change that, to make it high-risk and no profit."
Mr Hyland said the quality of official approaches to slavery varies around the country.
"There is inconsistency across the UK in a number of areas. That's training, police response, partnerships and to some degree the victim care," he said.
"In some parts of the country you see really first-class models and then in the same regions you see where there are systems and processes which need extensive improvement."
He called on police forces to make sure tackling slavery is positioned as "one of the highest priorities".
Children who should be at school are tasked with pickpocketing, shoplifting and begging streets
"I've been around the country now and met lots of police chiefs, most of whom are very committed. Yes, they have to consider their budgets and resources but I want to make sure the first thing they are talking about is the victims and catching the criminals."
Earlier this year the Modern Slavery Act was passed into law. It increased the maximum sentence for the worst offenders to life in prison and introduced a tough asset confiscation regime.
Mr Hyland described the legislation as a "watershed".
"It gives law enforcement more flexibility, more powers, it gives the victims far more protection through the criminal justice system," he said.
He stressed the UK must work to eradicate slavery at home and abroad.
"Over the next decade I think we can really bring the figures of modern slavery down to where it is almost eradicated from our shores," he said.
"We have a responsibility to help the international community to do the same. Clearly, that has enormous challenges.
"We need to really change the whole culture towards this activity in the UK and start on the process to eradicate it. It's an enormous task because it's got so many different faces."
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Karen Bradley, Home Office minister for preventing abuse and exploitation, said: "Kevin Hyland, the country's first Anti-Slavery Commissioner, is already playing a crucial role in raising awareness of modern slavery and driving our response to this appalling crime.
"The Modern Slavery Act - the first of its kind in Europe - gives the police the tools they need to ensure that perpetrators are brought to justice and enhances protection for victims who must be recognised for their bravery in coming forward, given the harrowing experiences they have endured.
"For too long, this was a hidden crime.
"Now, we have succeeded in bringing modern slavery out into the open, we are determined to eradicate it. There is no place for modern slavery in today's society."