Turning around Stoke-on-Trent's inadequate children's services may take more than two years, a review said.
A rise in children going into care and an overspend "by £8m for a number of years" were among several issues raised by a government trouble-shooter.
The council must partner with a better performing authority by 31 January.
The authority said it was committed to making improvements and recognised the scale of what was required following the four-month review.
The findings of Eleanor Brazil, appointed by the government as the council's children's services commissioner, came after Ofsted inspectors rated the department as inadequate in March.
She criticised previous leaders for not acting on recommendations to improve after a critical Ofsted report in 2015 and a local government association review in 2016.
That followed an external body being brought in to run the department from 2007 to 2010 due to poor standards of care.
'Make this happen'
Other concerns raised by Ms Brazil included workers with caseloads in excess of 100 children, care plans not progressing due to a 12% rise of children coming into the system and a lack of support for foster carers.
She said the "scale of change needed is significant and is likely to take at least two to three years", but highlighted concerns that financial pressures on the council "may get in the way" of the need for "the political drive" to make the changes.
Earlier this month, the council said up to 200 jobs were to be axed to free up money to invest in the department.
Council leader Abi Brown said she welcomed the help and being allowed to retain responsibility for the department.
"We know the challenge we are facing is significant, but we are determined to provide the best possible support we can for the children of Stoke-on-Trent and will do all that is needed to make this happen," she said.
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