The court advisory service, which assists children and families across the capital's 33 local authorities, was given the inspectorate's lowest grade on six counts.
Inspectors said that the service's overall effectiveness, performance management, engagement with users, responsiveness, attention to equality and diversity issues, complaints handling and ability to improve outcomes for children were all inadequate.
Families were forced to wait too long to be supported by Cafcass staff, whose poor communication with families was another area of concern. "Assessments are not shared consistently with children and families," the inspectors' report says.
"Complaints handling and the contribution by Cafcass to the promotion of improved outcomes for children and young people are inadequate, in particular in private law, where the impact of family breakdown on health, education and economic outcomes for children is not well considered or addressed," the report adds.
Inspectors do take into account that Cafcass has already sought to improve the service and reduce delays, judging the body's capacity to improve as satisfactory.
Action taken involves developing a "duty" triage-style system, to speed up cases and prioritise those involving the most vulnerable families.
Vince Clark, Cafcass operations director for the south, said: "From our own self-evaluation we were aware that we had to improve in a number of areas. Ofsted acknowledges the progress made to date."
