Ofsted inspectors said there was no assurance that vulnerable children were safe or “effectively protected” due to inadequate services at Dudley Council.
The authority had been aware of the deficits for “some time” but had not acted quickly enough, they added.
Dudley Council said it would “continue at pace” to radically improve services.
Inspectors who reviewed services in January and February rated children’s social care services inadequate in four out of five areas, including children who need help and protection, leadership and adoption performance.
They found no progression in most areas rated as “requiring improvement” in 2012.
The report said: “The long period of inaction means that many services have deteriorated since the last inspection.”
‘Extremely low base’
Inspectors highlighted 21 particular cases about which there were “serious concerns”, including children not receiving adequate levels of protection and those waiting for “unacceptably long periods” to see social workers.
Ofsted said the council was making concerted efforts to “tackle a legacy of deteriorating services”.
But it said the Labour-run authority was “working from an extremely low base” and the required improvements would take “a significant time”.
Too many children had experienced delay and “further risk” because the authority was “too slow to decide what needs to change in a family if it is going to be safe for children and young people to remain,” inspectors said.
“Neglect is a significant feature in Dudley, but there is no partnership-wide strategy to address this,” they added.
The report said support for those at risk of being sexually exploited was improving, and there was “a good range of services” for tackling domestic abuse.
However, it found children at risk of sexual exploitation did “not have effective safety plans in place”.
A children’s services commissioner will now work with leaders at the council to make further recommendations.
Official figures have shown more councils being rated inadequate or in need of improvement than were rated good or outstanding.
Since the start of the year, according to the Ofsted website, there have been four councils told their children’s services were inadequate and five that needed improvement. A further four were good and two were outstanding, Westminster City Council and the Royal Borough of Kensington and Chelsea.
Dudley Council chief executive Sarah Norman said: “We fully accept Ofsted’s findings and the inspection has confirmed our own self-assessment of the services and we will continue at pace to rectify the issues.”
Mike Wood, Conservative MP for Dudley South, said services are “clearly getting better” since new staff were hired last year, but said the conclusions from the Ofsted report were “terrible”.
“Obviously what it highlights is quite how little has been done to improve the care of vulnerable children over the last four years, and that’s of huge concern to everyone living in Dudley,” he said.
Ian Austin, Labour MP for Dudley North, has been approached for comment.