An Ofsted focused visit of children in need and child protection services at St Helens Metropolitan Borough Council last month found provision had deteriorated since the previous inspection in November 2014, when the department was rated "requires improvement".
The decline in services is despite "commitment and financial investment from political leaders", the inspectors' letter states.
In June, the council appointed Professor Sarah O'Brien as director of people. She replaced Mike Wyatt who had held the post since early 2016.
The inspectors' letter states that shortly after the council had been notified about the inspection, O'Brien warned Ofsted that she had identified "serious shortfalls in practice".
"The focused visit confirmed that entrenched cultural, management and social work practices are negatively impacting on children's outcomes," the letter states.
"There are poor threshold decision-making and delays when escalating children's cases to child protection plans and also to pre-proceedings processes.
"The local authority fails to address poor and harmful living conditions for too many children - which means [they] live for too long in circumstances in which they are experiencing ongoing risk and harm."
In addition, inspectors found that weaknesses in practice were not tackled because management oversight "at all levels in children's social care is poor".
Where inspectors identified drift and delay in cases the authority failed to take "sufficiently robust action", despite clear risks to the children, the letter added.
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Ofsted has identified three areas for the council to take urgent action - tackling delays in escalating cases, improving management oversight, and introducing better quality assurance and audit processes to help identify and challenge poor practice.
The council has been ordered to submit a final action plan by 3 December and a draft plan by 10 September - the latter as a result of an amendment to the Ofsted inspections framework that will take immediate effect.
The letter identified improvements in the recruitment and retention of social workers and the support provided to newly qualified practitioners. It also recognised that the council was "open" with inspectors about its deficiencies and what it needs to do to improve.
O'Brien said: "We accept the findings of this letter and are determined to make the necessary changes and improve outcomes and experiences for our children and young people.
"Since our last inspection in 2014 we have been working hard to make improvements, however they have not delivered the impact we wanted.
"The letter has shown the areas in which action is needed, and we have taken immediate steps by setting up an improvement board, to oversee delivery of the action plan. The first meeting of the improvement board has already met in shadow form, and we will be working together with colleagues from the region, Ofsted and the Local Government Association to make further improvements.
"The care and safety of vulnerable children and young people is an absolute priority for this council and we will do whatever it takes to make sure that we are providing them with the high-quality services that they deserve."