Nine in 10 councils across England are placing children in unregistered homes which can charge up to £30,000 a week per child.
The findings, published in Ofsted's annual report, describe how local authorities often cannot find places in registered homes to cater for children's needs, despite the number of care homes being at a record high.
Ofsted chief inspector Sir Martyn Oliver described the situation as an "ongoing national scandal".
The Children's Homes Association (CHA), which represents residential childcare providers, said there is no clear national plan and that it is up to the government to "fix the system".
