The social care Wakefield Council provides for children has been deemed 'inadequate' in all areas by Ofsted.
A report published today said there were "serious and widespread failures" across children's services in the district.
It said there were cases where appropriate action had not been taken to help and protect children and serious delays in ensuring the needs of vulnerable children in care were met.
Inspectors said: "Services for children in need of help and protection in Wakefield are inadequate because there are widespread and serious failures that leave children at risk of harm or living in harmful situations for too long."
Responding to the report, Coun Peter Box, the leader of the council, said: “This service has let down children and families of this district and that is completely unacceptable.
“I have been clear for several months that very serious issues need to be addressed, and quickly.
“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.
“This is now changing and our priority is to see this make a difference to our children and their families quickly."
The report comes after a full inspection of children's services by Ofsted from June 4 to June 15 and follows a damning visit in February, in which it was identified that children were not being protected and that there were serious gaps in the management of the service.
In the last full inspection, carried out in 2016, children's services were judged to require improvement.
Today's report said: "Many of the identified improvements have not been delivered and previously good services have deteriorated."
In March, a new corporate director for children and young people - Beate Wagner - was appointed.
A new service director for children's social care is also due to join the council on August 7.
Coun Box also allocated £1m for a transformation action plan and has invested an extra £3.5m in the service's annual budget.
Merran McRae, the authority's chief executive, said a "rapid review" of every child in the council's care was being undertaken.