The watchdog has now published the results of unannounced visits to 18 local authorities, nine of which have been told to improve urgently. These can be viewed by clicking on the council names below.
Leeds, Calderdale, Cambridgeshire, North Yorkshire, Cornwall, Hounslow, Redbridge, Warrington and East Sussex were all classed as having at least one "area for priority action".
In Leeds, inspectors found that social workers left seven children at risk of serious harm. The council's response to child protection referrals was deemed inadequate and children were not always seen by social workers, even when there were concerns about their safety.
In the London Borough of Redbridge, Ofsted found that families were left waiting to access services, because a high proportion of initial and core assessments were completed late.
Blackpool, Sandwell, Buckinghamshire, Wolverhampton, Bromley, Hillingdon, Swindon, Greenwich and Enfield fared better in their no-notice inspections.
None of the councils were asked to improve practice urgently, although they were advised of "areas for development".
Ofsted found that Swindon Council responded to child protection concerns promptly. Senior managers and elected members in the authority also "regularly scrutinise safeguarding arrangements".
In the London Borough of Hillingdon, inspectors found that the morale of frontline staff was high and management took effective action to help reduce workload pressure on staff.
Rosemary Archer, director of children's services in Leeds, said: "We recognise and fully accept the seriousness of the inspection's findings. For this reason we responded within hours to the concerns of the inspectors."
She added that the council has now undertaken a major review of previous cases to ensure that no children are being left at potential risk of serious harm.
A spokeswoman for Redbridge Council said: "As with all inspections there are always areas for development and we take them on board. However, all of these identified areas for development were already outlined in our action plan prior to the inspection, therefore were being implemented."
She added that the council is investing an additional £400,000 into social services to cope with the rise of referrals since the Baby P case.
Garry Perkins, lead member for children's services at Swindon Borough Council, said the results of their snap inspection are testament to the good work of social care staff.
As CYP Now reported, Ofsted had been set to use the phrase "serious concerns" to describe significant problems found at no-notice inspections.
But the watchdog is now referring to concerns as "areas for priority action", after directors of children's services claimed the term "serious" was misleading.
David Williams, chair of the Association of Directors of Children's Services' standards, performance and inspection committee said: "Where areas are identified that put children at risk it is right that these should be highlighted so that local authorities can take appropriate action immediately and that progress can be monitored.
"We will be looking at the inspection findings in more depth prior to meeting with Ofsted to review the first inspections under the new framework to ensure that the evaluation of inspectors' findings is consistent and appropriate."
