Children are being left "at risk of harm" because of the Government's failure to develop a "credible" plan to improve protection services, MPs have warned.
The influential House of Commons Public Accounts Committee (PAC) said progress had been "too slow" in the five and a half years since the Munro Report, which called for a major overhaul of the system.
In a new report, the cross-party committee accused ministers of "complacency" on the issue and called on the Department for Education to publish detailed plans to transform child protection services.
"Urgent" action is needed to end wide variations in the quality and consistency of help available to vulnerable children in different parts of the country, it said.
Professor Eileen Munro was commissioned by then education secretary Michael Gove to conduct a review of child protection services. Her 2011 report called for social workers to be freed from excessive bureaucracy and centrally imposed targets and regulations to give them more time for face-to-face work with families and at-risk youngsters.
In response, ministers said they were adopting Prof Munro's principles for the service and promised to "move towards a child protection system with less central prescription and interference, where we place greater trust and responsibility in skilled professionals at the front line".
But in the new report, the PAC said the Government still had "no credible plan to improve services and grow a quality social work force".
Less than a quarter of services were rated "good" by Ofsted, which the committee described as "by no standards" an improvement.
The PAC called on the DfE to "set out detailed plans, including a timetable and resources, for how it will work with local authorities to transform services".
Ministers need to explain how they will ensure that all children have equal access to high-quality services and how they will attract more "high-calibre people" to social work, said the report.
And it called for more resources for Ofsted to conduct quality checks, and greater readiness to intervene to nip local problems in the bud.
Committee chair Meg Hillier said improvements were "woefully overdue" and it was "completely unacceptable" that so little progress had been made since the publication of the Munro Report.
"Government complacency over improving children's services must end now," said Ms Hillier.
"There are nearly 800,000 children in need of help or protection every year – children who for far too long have been let down by the support available."
The Government's target of 2020 to transform the system "better serves Whitehall than it does vulnerable young people in need of help", said Ms Hillier.
"There is much at stake here and by the spring I will expect the Department for Education to provide us with clear plans to drive the improvements that are woefully overdue."
A Department for Education spokesman said: "We have a relentless focus on keeping children safe, and it is wrong to suggest otherwise. This year we published plans to deliver excellent children's social care across the country, and through new legislation are further strengthening protection for the most vulnerable children and transforming the support available to them.
"We take tough action where councils are failing children, stepping in to make sure improvement plans are taken forward as a matter of urgency."
Anne Longfield, children's commissioner for England, said: "It is critical that every child in the country is properly protected from harm – there should be no room for complacency. No child's safety should be dependent on a postcode lottery of spending and provision.
"I have previously raised concerns about the lack of progress in improving protection for children in many parts of the country and we need to see drastic and rapid improvement.
"Although authorities are facing challenges such as increased referrals, excellent performance ought still to be possible in every area.
"There needs to be far more emphasis on learning and implementing lessons from the best performing authorities. The huge disparities in average social work spend per child do not correlate with local authority performance, so part of the solution must lie in spending money more wisely."
The chairman of the Local Government Association's children and young people board, Richard Watts, said councils had faced "significant" funding cuts in a period when numbers of cases increased by 65% in England from around 1.2 million in 2007 to more than 2 million in 2013/14.
"With such a big rise in demand for services, it's vital that local authorities have the resources they need to keep children and young people safe," said Mr Watts.
"Inadequate" ratings handed down to child protection departments by Ofsted often resulted in "resignations by both councillors and officers, vilification in the media, and an uphill battle to recruit new social workers", making it "incredibly difficult" to improve standards, he said.
"We would like to see Ofsted playing a far more active role in supporting improvement, including an inspection framework that recognises something as complex as children's services cannot be reduced to a one-word rating," said Mr Watts.
"We can never be complacent when it comes to the safety of children and young people, but we must take care that in our rush to improve, we don't lose sight of the unreported excellence of social workers across the country, whose tough decisions and swift actions are saving children's lives every day."