Ofsted, which inspects children's social care including child protective services, said more needs to be done to address "incompetent and ineffective" leadership in children's services.
Some managers do not have a "firm understanding" of what constitutes good practice, the watchdog said.
In its first stand-alone social care annual report, Ofsted said that children's services need "strong and stable leadership" to improve the services that provide help, care and protection to children at risk of abuse or neglect.
Inspectors found that in the weakest places many "basic acceptable practices" were not in place and some authorities were criticised for poor coordination with health workers, police and schools.
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Labour strongholds tended to remain so - apart from a major loss for the party in Dudley.
Labour suffered a "terrible" defeat in Rhondda but has already won 16 Welsh Assembly seats as it looks to record a fifth term in office.
UKIP have made further gains in the local elections in the Anglia region and the Conservatives have won back control in Peterborough.