"Significant weaknesses" in special educational needs and disability (Send) services have created "inequality and inconsistency" across Essex, according to an inspection report.
In some areas children were waiting up to two years for autism assessments, the joint CQC and Ofsted report found.
One mother said Send provision was "in crisis".
Essex County Council and five clinical commissioning groups (CCGs) said they accepted the report's findings.
The service helps under-25s who need additional help with schooling and creates educational care plans.
But many aspects of the service were criticised, including issues in commissioning between the council and the CCGs, and variations leading to "inequality, inconsistency and unacceptably long waiting times" for families.
Sarah Barker, whose son Rowan uses the service, said she had problems in getting different teams to communicate with each other, leading to delays in Rowan's educational health care plan.
"I think the report essentially shows that SEND provision in Essex is in crisis and has been for some time," she said.
"I've spoken with dozens of other parents who have had similar negative experiences, some of them having to battle for years to get the right support for their children."
Essex County Council works with CCG groups in Basildon and Brentwood, North East Essex, West Essex, Mid Essex and Castle Point and Rochford to proved Send services across the county.
Ray Gooding, the council's cabinet member for education and skills, said: "Prior to the inspection we had already begun an improvement and development journey as our own self-assessment had identified significant areas that require improvement; this was confirmed by the inspection."
He said the council and CCGs were working to address the concerns.