A Peterborough children services backlog of more than 230 cases put youngsters at risk of harm, says a watchdog's report.
The Care Quality Commission and Ofsted report rated three key city council children services areas "inadequate".
It says children were put at risk because more than 230 cases were waiting to be allocated and assessed when the inspection started.
The city council apologised, but said the matter had been dealt with.
The report says the backlog of cases over a four-week period from December 2009 to January 2010 and the lack of assessment left "a significant number of children at risk".
The report adds that the children services' effectiveness dealing with Section 47 cases - where local authorities must make inquiries about children suspected of being at risk - was "variable".
"There are long delays in undertaking Section 47 inquiries, a lack of managerial oversight and examples of poor judgement by previous front line managers leaving children at risk of potential harm," it says.
The report calls for immediate action to ensure that Section 47 investigations are "timely and... clear" and the quality of assessments is improved.
Peterborough City Council stressed that out of 33 categories, 21 were judged as good and nine were judged as adequate with three judged inadequate.
A spokesman pointed out the report said the council's Director of Children's Services, John Richards, had already identified the backlog and was already taking action to improve safeguarding services for children.
Mr Richards said: "I am sorry for the shortcomings in the service during this four-week period in December and January that led to the backlog of cases being allocated.
"However as soon as I discovered it I took robust action and the individuals responsible are no longer working for us.
"I would like to reassure the public that this was a temporary issue, that was dealt with."
He added: "Safeguarding all children in Peterborough is, and always has been, my number one priority."