The quality of help and protection offered to at-risk children across England is "unsatisfactory and inconsistent", a report has found.
The chair of an influential House of Commons committee said it was "horrifying" that more than three-quarters of local authority child protection services were found to be inadequate or requiring improvement, despite annual spending of £1.8 billion on children's social work.
Spending watchdog the National Audit Office (NAO) found that demand for help and protection is rising, with a 124% increase in the past 10 years in the number of serious cases where a local authority believes a child may be suffering, or likely to suffer, significant harm.
In March 2015, 3% (391,000) of children under the age of 18 in England were assessed as being in need of help or protection, with the most common risks resulting from domestic violence and mental health concerns. Some 635,000 children across England were referred after seeking help or having concerns raised by teachers, doctors, health visitors or members of the public during 2014/15.
Despite the Department for Education recognising the existence of a problem in 2010 and taking action on the recommendations of the following year's Munro Review on child protection, the services offered to children by local authorities are still "not of good enough quality", the NAO report found.
The auditors found that the problem was "systemic", rather than resulting from failings or shortages of cash in particular local areas. The £2,300 average annual spend on a child in need has increased slightly over the past three years, but the figure varies wildly from £4,970 in the highest-spending authority to £340 in the lowest, without the NAO finding any relation to quality levels.
Almost 80% of councils inspected by Ofsted over the past three years failed to meet the "good" rating for support to children needing protection, said the NAO. Some 26 out of 152 child protection departments are subject to Department for Education intervention after being rated "inadequate".
But the NAO found that good performance was not related to levels of deprivation in an area, numbers of children covered by the authority, or the amount spent on children in need.
Auditor general Amyas Morse said: "Six years have passed since the department recognised that children's services were not good enough. It is extremely disappointing that, after all its efforts, far too many children's services are still not good enough.
"To achieve its new goal of improving the quality of all services by 2020, the department will need to inject more energy, pace and determination in delivering on its responsibilities."
Meg Hillier, who chairs the Commons Public Accounts Committee, said: "Children in need depend on child protection services to get it right for them where other adults in their lives have failed.
"It is horrifying that over three-quarters of local authorities' child protection services are inadequate or require improvement to be good."
The NAO found that thresholds for assistance varied from area to area, meaning some children were left at risk while others were inappropriately referred for help.
The Department for Education and local authorities lacked the data needed to understand which approaches would provide the most effective help and protection.
The report found that the department does not intervene early enough in failing children's services and faces "significant challenges" to transform standards by its target date of 2020.
Richard Watts, chair of the Local Government Association's children and young people board, said: "In 2008, 78% of children's services were rated good or outstanding by Ofsted. It is notable that this figure has now dropped below 25% over a period in which child protection reform and improvement has been largely removed from local government and increasingly centralised within Whitehall instead.
"It's vital to examine how DfE initiatives imposed on local authorities, such as children's services trusts, are evaluated to check whether they are doing a better job of looking after vulnerable children, and use that evidence to develop future initiatives in partnership with councils."
An NSPCC spokesman said: "This damming report re-exposes some gaping holes in local authorities' ability to deliver effective services and protect vulnerable young people, while also revealing the vast extent to which support varies across the country."
A Department for Education spokeswoman said: "Keeping children safe from harm is an absolute priority for this Government, which is why in July this year we published plans to deliver excellent children's social care - strengthening protection for the most vulnerable children and transforming the support available to them.
"We are taking tough action to drive up standards in children's services across the country, stepping in when councils aren't doing well enough and linking them up with better performing local authorities to share best practice. We have also cut red tape so that social workers can spend more time actually supporting families."