Calderdale Council's children's services department is not doing enough to safeguard young people, an Ofsted report has found.
The authority, which covers towns such as Halifax and Elland, was one of 12 councils in England rated as performing poorly by the education watchdog.
Ofsted said it found serious weaknesses in the department, including in the quality of assessments and records.
The council pledged to continue making improvements to children's services.
In its report, Ofsted also said the council's "pace of change had been slow in addressing known areas of weakness" and it found "poor performance in social care fieldwork teams and in management support for staff".
The report said: "More positively, [inspections] found a culture of openness and a willingness to accept external challenge.
"It also found there were some positive local partnerships, notably with health, but that these were not consistently good. Capacity for improvement was judged to be adequate."
The council said it would continue to make children's services a "top priority" and had recently made improvements.
It said a backlog of unallocated referrals had been cleared and 30 staff had been given further training.
Councillor Olwen Jennings, cabinet member for children and young people, said: "The safety of Calderdale's children is a top priority and I am confident that everyone continues to work hard in order to address the issues already identified.
"Clearly there are concerns in a number of areas, but there is also a lot of great work going on to achieve positive outcomes for young people."