Ofsted have found Buckinghamshire County Council's children's services to be 'inadequate' in a new inspection report published on Friday.
The inspectors found performance unsatisfactory in leadership, management and the way the service deals with increasing volumes of work and record keeping.
The report highlighted 261 children who needed help but had not been allocated a social worker, despite many living in "neglectful and unsafe circumstances".
Inspectors also said key areas of social work had "fundamental weaknesses" and it took the authority too long for social workers to see children most at risk.
It said "immediate action" must take place to ensure vulnerable children and their families have adequate access to support.
Angela Macpherson, cabinet member for children's services at the council said: "We must get back to a 'good' rating or better as soon as we can.
"We are already finalising a new action plan in response to the report so it's crystal clear to everyone how and when these improvements will be delivered."
To download the report, visit: http://www.ofsted.gov.uk/local-authorities/buckinghamshire