Ofsted has found ‘serious failures' in services for young people in Devon.
Devon CC's children's services have been rated inadequate after an Ofsted inspection in January.
The inspectors' report said that senior leaders did not know about the extent of the failures to protect some of the most vulnerable children and young people from harm.
Ofsted found that some children who have suffered chronic neglect and emotional abuse were being left with their families for too long because social workers and their managers ‘lack clarity' about when to turn concerns into legal action.
The report concluded: ‘A succession of inspection reports over the last seven years have emphasised that the local authority needs to have a better understanding of its strengths and weaknesses and to take robust and swift action where improvements are necessary.'
Devon's deputy leader, James McInnes, accepted the findings and announced that Devon will be working closely with Cornwall Council to bring about immediate improvements.
He said: ‘Obviously, I am bitterly disappointed that we have let any vulnerable young person down.
‘I have every confidence in our senior managers and frontline staff to act fast and do what is needed to turn things around.'