Inspectors found a series of cases where children known to Wakefield Council's children's services department were at risk, but the situation had not been identified and appropriate action had not been taken to help protect them.
The inspection, which took place in June, was prompted by a two-day "focused visit" to check on provision, which was conducted in late February.
That found that social workers are under pressure from high caseloads and suffering from "low morale". Inspectors found high numbers of unallocated cases and that the council is at loggerheads with partners on how child protection thresholds are applied.
Under the Inspection of Local Authority Children's Services (ILACS) system, introduced earlier this year, local authorities that are rated "good" or "outstanding" receive a short one-week inspection every three years while those graded as "requires improvement" will get a standard two-week inspection every three years. These councils will also have at least one "focused" visit between inspections.
Wakefield was last inspected under the old single inspection framework in July 2016, in which it was rated as "requires improvement to be good", meaning it was not due to have a further inspection until 2019.
Wakefield's director of children's services John Wilson left last November and his role was filled on an interim basis by Eleni Ionannides, who is now interim DCS at Croydon Council. Wilson's permanent replacement, Beate Wagner, joined in February.
Ofsted's inspection report notes that Wagner has started to recruit a new management team to turn around children's services. This includes the appointment of Vicky Schofield, who joins next month as service director for children's social care from Rotherham Council, where she leads the first response service.
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The report also recognises that the council has allocated extra money for improvements. This includes £1m to put in place an improvement action plan and a £3.5m increase in the children's services annual budget.
But inspectors are concerned that "the plan is not based on systematic evaluation of the current service weaknesses, or on a full understanding of the experiences of children".
Their report also details how since their 2016 visit, concerns have not been acted on and previously good services have deteriorated.
Inspectors found that all aspects of children's services are "inadequate", covering child protection, support for children in care and care leavers, leadership of the service and overall effectiveness.
"There are serious and widespread failures across children's services in Wakefield," states Ofsted's inspection report.
"Inspectors found cases of children throughout the services for whom risk had not been identified and where appropriate action had not been taken to help and protect them. There are serious delays in achieving permanence for children in care and in ensuring that their needs are met."
It adds that "the absence of case recording has become a pattern in many teams" which managers are not consistently challenging.
They are also concerned that "a significant weakness across Wakefield's social work practice is the failure to recognise and respond to risk".
Wakefield Council leader Peter Box said that the council's children's services has "let down children and families".
"The findings of this latest Ofsted inspection confirm much of what we already knew. It is clear that, for far too long, we had a system unable to support frontline social workers, too few managers to supervise and support, and not enough social workers to meet the needs of our children," he said.
"This is now changing and our priority is to see this make a difference to our children and their families quickly.
"I can assure residents that Ofsted's recommendations are being fully actioned and the council will ensure that every resource possible will be put into transforming these services, putting children at the very heart of everything we do."
Merran McRae, the council's chief executive, added: "We are acutely aware of the many significant challenges we face and are absolutely ready and willing to learn and improve.
"We have immediately begun tackling the serious concerns about some children being at risk.
"A team of highly-experienced, external professionals have been recruited and are carrying out a rapid review of every child in our care. Urgent action will be taken if any child is identified as being at risk."
When the ILACS framework was launched, Ofsted described it as "more proportionate, risk-based and flexible" than the previous single inspection framework, which was introduced in November 2013.
Wakefield is the first council to receive its lowest rating under ILACS. Although Surrey County Council was rated as "inadequate" in May, after the new framework's launch, that was a re-inspection under the single inspection framework.
