Children's services overspend hit £605m among 75% of English councils in 2015-16
Three-quarters of councils in England overspent on their children's services budgets by more than half a billion pounds, council leaders have warned.
Figures for the 152 English councils which provide children's social care services showed 113 (75%) spent more than they had budgeted for, while 38 spent less in 2015/16, analysis by the Local Government Association suggested.
One council did not submit any data.
Councils in England spent £8.303 billion on children's social care in 2015/16 compared with a budget figure of £7.698 billion, surpassing their budgets by £605 million in order to protect children at immediate risk of harm, the LGA said.
Demand for children's social care support rose almost 140% in a decade, with more than 170,000 children subject to child protection inquiries in 2015/16, compared with 71,800 in 2005/06, the LGA data found.
Over the same period, the number of children on child protection plans also increased by almost 24,000, it added.
The LGA argued the figures showed the "sheer scale of the funding crisis" faced in children's social care, as it warned of a £2 billion funding gap expected by 2020.
Resources were being used to provide urgent assistance for children and families, leaving "very little" to invest in early intervention, the LGA said.
Councillor Richard Watts, chairman of the LGA's Children and Young People Board, said services for the care and protection of vulnerable children were now, in many areas, "being pushed to breaking point".
He said: "The fact that the majority of councils are recording high levels of children's services overspend in their local areas shows the sheer scale of the funding crisis we face in children's social care, both now and in the near future.
"Councils have done everything they can to respond to the growing financial crisis in children's social care, including reducing costs where they can and finding new ways of working.
"However, they are at the point where there are very few savings left to find without having a real and lasting impact upon crucial services that many children and families across the country desperately rely on.
"With councils facing a £2 billion funding gap for children's services in just three years' time it is more important than ever that the Government prioritises spending in this area.
"There is no question that early intervention can help to limit the need for children to enter the social care system, lay the groundwork for improved performance at school and even help to ease future pressure on adult social care by reducing the pressure on services for vulnerable adults."
Shadow communities secretary Andrew Gwynne said the LGA's analysis shows that children's social care services are "on the brink of collapse".
He said: "Councils have worked hard to make savings, but they are running out of options.
"We all want every single child, no matter their geography or family situation, to receive the best care and protection when needed, but right now, that is not happening.
"This is yet another sign of the Government's misplaced priorities - they can find a £1 billion bung for the DUP to stay in the job but are failing to provide the funding children's services desperately need."
A Government spokesman said: "Every single child should receive the same high-quality care, support and protection, no matter where they live.
"Councils will receive more than £200 billion for local services, including children's social care, up to 2020. This is part of an historic four-year settlement which means councils can plan ahead with certainty.
"Councils are doing excellent work, spending nearly £8 billion in total last year on children's social care, but we want to help them make sure they do even more.
"That's why we set up the £200 million children's social care innovation programme to help them develop new and better ways of delivering these services."