Child social care is "inadequate" or "requires improvement" in three-quarters of the local authorities in England inspected by Ofsted last year.
Ofsted made more than 5,000 inspections of children's homes and other social-care provisions run by 43 councils.
The watchdog says demand for children's services has been rising continually for the past seven years.
Local authorities called for better funding for children's services but also raised concerns about Ofsted.
Ofsted says it found "compelling evidence" that children and young people were often waiting too long for the help they needed.
Debbie Jones, Ofsted's national director for social care, told BBC Radio 4's Today programme that preventative services were not being put in place early enough for children who were being abused or neglected.
"A young person is left too long at home without the services they need," she said. "Get in early, get in quick. It can make a life-long difference."
Scandals involving local authorities in Rotherham, Rochdale and Oxfordshire have raised the political profile of social care for children.
Last week, the prime minister said children in the UK had suffered sexual abuse "on an industrial scale", with authorities failing to tackle the problem over decades.
These were the first inspections carried out under a new framework introduced in November 2013.
- children's homes
- independent fostering agencies
- voluntary adoption and support agencies
- residential family centres
- holiday schemes for disabled children
The inspections are carried out over three or three-and-a-half year cycles.
Seven local authorities were found to be inadequate in this round:
Ofsted found "little evidence of decisive action to keep children and young people safe" in these areas.
Inspectors highlight a number of problems common to all seven, including:
- instability in leadership
- high staff turnover rates
- poor management oversight
- poor assessment and planning
A further 26 local authorities "require improvement".
Ofsted says that while there are no "widespread or serious failures" in these areas, the authorities are "not yet delivering consistently good protection".
Across many of these councils inspectors found a "lack of co-ordinated and early intervention", along with inconsistent management oversight of, and support for, social workers.
Ofsted says it is "committed to supporting these local authorities to improve", even running a series of "Getting to Good" seminars that focus on identified needs.
Inspectors did find some evidence of improvement, with Essex and Cambridgeshire - which were both previously judged inadequate - now found to be good.
Ten authorities out of the 43 inspected in this round were found to be good.
Ofsted points out that there has been a 12% rise in the number of child-protection investigations last year and the number of referrals to children's care services increased by almost 11%.
'Marking its own homework'
The Local Government Association (LGA), which represents local authorities, says its members are coming under increasing pressure.
David Simmonds, the chairman of the LGA's children and young people board, believes children's services are "creaking under the strain".
"Protecting children is one of the most important jobs councils do, and this report restates the pressure the system is under," he said.
"In an NHS system failing to cope with winter pressures, the government recently pledged £2bn to alleviate the crisis.
"We need Whitehall to redress the balance and give us adequate resources we need to get on with the vital job of protecting children.
"High-profile crimes of abuse and neglect have brought sharply into focus the need for vigilance.
"As a result, there are rightly thousands more children on the radar of social services now.
"But this is in a climate where councils have faced cuts to their budgets of 40% since 2010."
Mr Simmonds stressed the need for help from the public in protecting vulnerable children.
He said: "We need a million eyes and ears looking out for our young people.
"Far too many times social workers hear of abuse too late, when we need to be intervening earlier."
He also questioned the credibility of the inspections.
"There is a need for an urgent, back-to-basics review of Ofsted, as there are big question marks over the quality of judgements following what has happened in Rotherham and Birmingham, among others," he said.
"We are concerned that by trying to be an improvement agency as well as an inspectorate, Ofsted is marking its own homework."