Millions of children in England are living vulnerable or high-risk lives, a report has revealed.
The Children's Commissioner's report on measuring the number of vulnerable children found that 580,000 children - the equivalent to the population of Manchester - are so vulnerable the state has to step in and provide direct care, intervention or support.
It also found almost 670,000 children are living in families that have vulnerabilities, including over 15,499 children living with an adult receiving alcohol treatment and nearly 11,624 living with an adult in drug treatment.
The report, published by the Children's Commissioner for England, Anne Longfield, revealed that around 805,000 children aged five to 17 suffer mental health disorders.
An estimated 46,000 children are thought to be part of a gang, while 1,200 children are newly identified as victims of modern slavery every year, it found.
However it is thought the figures are likely to be an underestimate as many children are "invisible" because they are not reported to the services, or because of gaps in the data.
Ms Longfield said: "It is shocking that half a million children - a number equivalent to the entire population of Manchester – need direct intervention or care from the state because they are living vulnerable lives.
"On top of that there are many hundreds of thousands of other children growing up in potentially high-risk situations.
"Yet even more shocking is that this is only the tip of the iceberg. The actual numbers are likely to be much higher.
"The truth is nobody knows the exact number of vulnerable children. We can trace in minute detail the academic progress of a child from four to 18 and beyond, but when it comes to describing and assessing the scale of negative factors in a child's life which will hamper their progress, we are floundering."
Currently different agencies involved with children may apply different criteria to the term "vulnerable", and sometimes the same criteria is used but the term "vulnerable" is not.
The launch of the report is the first stage in a programme of work on children's vulnerability.
It will start by tackling the confusion over what this means, and the commissioner will now consult broadly on the definitions and develop a framework that can be used widely.
The Children's Commissioner argues in her report that Government must do more to collect better data, and questions how effectively the problems outlined in the report can be tackled if departments and agencies do not know how many children are affected or cannot agree on how to define and therefore identify them.
Robert Goodwill, Minister for Children and Families, said: "Every single child should have their voice heard and receive the care and support that they need to realise their potential.
"Across government, we are taking action to address this issue - whether through reforming children's social care, prioritising mental health, or better protecting victims of domestic violence and abuse.
"For some of the most vulnerable, our new What Works Centre for children's social care will ensure social workers across the country are able to learn from best practice in keeping children safe.
"We recognise the scale of this challenge – and, while the number of children in need has remained relatively static since 2010, there is always more to do. We will look carefully at these exploratory statistics and I am looking forward to working with the Children's Commissioner as this important work continues."