Children with special educational needs and disabilities (SEND) in Hertfordshire wait too long to be assessed and helped, inspectors have found.
The service received the lowest rating possible, with some children found to have been more than 78 weeks for help.
Hertfordshire Local Area Partnership has apologised to families.
The report found a significant shortage of specialist education places and delays in diagnosis, as well as long wait times to access speech and language services.
"Too many children and young people with SEND have to wait too long for their needs to be assessed and for suitable provision to be put in place to meet their needs," the report said.
It said this had an impact on children and young people's experiences and limited their "chance of success".
The report also highlighted lengthy delays over education, health and care plans which are often of a "poor quality".
The inspection, completed over four days in July, found the rate of exclusions for children and young people with care plans was a "concern".
"A significant number of parents feel they have no alternative but to educate their child at home while they are waiting for a place in a special school to become available."
Hertfordshire County Council and the NHS Hertfordshire and West Essex Integrated Care Board are part of the local area partnership responsible for planning and commissioning SEND services.
It has been given 35 days to publish a "priority action plan" and a monitoring inspection will be carried out within 18 months.
The partnership said: "We want to say sorry to you, our families."
We understand the scale of the issues and know that too many children and young people with SEND and their families have not had the service and outcomes they need and deserve.
"We have already put in place strong improvement plans."