The leadership of North East Lincolnshire Council has pledged to protect and support young people after the local authority's children's services were judged ‘inadequate' overall.
Ofsted inspectors visited the area last month to conduct a full inspection of the department.
A report into the findings said there were ‘serious weaknesses' in key areas.
The council's children's services were graded ‘inadequate' in three areas: the impact of leaders on social work practice with children and families, the experiences and progress of children who need help and protection, and the experiences and progress of children in care and care leavers.
A Department for Education appointed commissioner will now work with the council.
Ofsted said services to children and families have ‘significantly deteriorated' since the council was judged to be ‘good' in 2017.
The report read: ‘This inspection has identified widespread weaknesses in assessment, planning and management decision-making, leaving many children at risk of harm.
‘Corporate leaders and senior managers do not have sufficient understanding about the quality of practice and the level of risk and need that children are experiencing.'
Council leader, Cllr Philip Jackson said: ‘We recognise and accept Ofsted's findings, but let me emphasise today that North East Lincolnshire Council is absolutely committed to continuing its journey of improvement and in doing so we will work with all concerned to keep our children safe.'
Ofsted's report comes as the council's director of children's services, Lisa Arthey, announced she was leaving the authority after almost two years – eight months of those as director.
In a statement, the council said that during Ms Arthey's directorship ‘significant inroads' were made into the changes required.
Portfolio holder for children's services, Cllr Ian Lindley said: ‘We will not shy away from this - instead we will rise to it.
‘The council is committed to work with the specialist commissioner, alongside partners, to accelerate an improvement plan.
'This three-year plan, implemented in 2019, is underway and is already seeing positive changes – a fact highlighted in the report, with praise in a number of areas.'
