The effectiveness of Birmingham City Council's arrangements for protecting children has been judged "inadequate".
A surprise inspection last month found some children had been left at risk of harm, a report from Ofsted said.
Council leader Sir Albert Bore said it made "very distressing reading", but some improvement had been recognised.
Birmingham Social Services was served with an improvement notice in 2009 following several child deaths, including the case of Khyra Ishaq.
Ofsted rated the council's child protection services inadequate in 2010 but a report from inspectors in October 2011 found they were improving.
The latest report said the Labour-run authority's arrangements for protecting children were inadequate in all four areas listed: overall effectiveness, the effectiveness of the help and protection provided to children, young people, families and carers, the quality of practice and leadership and governance.
The report said the inspection "found that too many children and young people are left for too long without a robust assessment, leaving some children at risk of harm".
Children and young people subject to child protection plans "do not always receive statutory visits from social workers", it added.
But Ofsted said the new director of children's services, Peter Duxbury, was "a key driver" in some recent changes "in taking forward improvements".
Sir Albert said it was "simply not acceptable" that services for children and families in the city continued to be poor.
He added: "We are clear about what needs to be done and we have already put systems in place, such as intensive training programmes so staff learn from our best managers.
"There must be significant improvement over the next 12 months."
Birmingham Safeguarding Children Board, a statutory partnership board with responsibility for co-ordinating work to safeguard children, said it accepted the report's findings that changes the board was leading "have not yet had a good enough impact on children's lives".
A Department for Education spokesman said: "There has been some progress but it's clear that there is still a huge amount to do before children's services are fit for purpose. We're working with Birmingham to make sure robust action is taken urgently."
A serious case review by the Birmingham Safeguarding Children Board into the death in 2008 of seven-year-old Khyra Ishaq, who was mistreated at a house in Handsworth, found it could have been prevented.
Her mother was sentenced to 15 years in prison after admitting manslaughter on the grounds of diminished responsibility.
Khyra's stepfather was jailed indefinitely after his manslaughter plea was accepted and must serve a minimum of seven-and-half years.
Elsewhere in the West Midlands, Sandwell Council's children's services were rated as inadequate in 2009, but an adequate rating was given after Ofsted inspectors returned in January this year.
Inspectors visiting Walsall in June 2012 rated safeguarding services as inadequate.
But Walsall Council said in the next few weeks it was likely to receive a type of improvement notice from the government showing it had taken steps towards making children's services better.