During a visit in November, Ofsted and health watchdog the Care Quality Commission found that senior leaders have been too slow to implement disability and SEN reforms and parents are often unclear about what support is available to them.
"A lack of strategic direction, frequent staff changes and changes to the roles and responsibilities of senior leaders have reduced the capacity of the local area to deliver the statutory reforms effectively," the letter states.
"Joint commissioning between healthcare services and the local authority was not established soon enough and is underdeveloped.
"As a result, inequalities in the quality of services and the provision for children and young people who have SEN and/or disabilities remain.
Inspectors said the local area's SEN and disabilities strategy is still at the consultation stage, which has "slowed significantly" the local area's work to plan and deliver high-quality services and provision for children and young people with SEND.
They also warned of "long-standing problems" in some services provided by the local area, pointing to young people having to wait for lengthy periods of time to have their needs assessed, and met, by clinicians and specialist health and social care practitioners.
"Families continue to experience unacceptably long waits for the assessment and diagnosis of children and young people with autistic spectrum disorder," the letter states.
Meanwhile, parents and carers were found to overwhelmingly report that the "local offer" is not easy to use and fails to provide the information they want and need.
"A significant number of families who shared their views with inspectors had not heard of the term ‘local offer' or did not know what it was," the letter states.
"Many of them do not know what help is available, where or how to access it. They are too reliant on self-help, and ‘ad hoc' or informal support from other parents and families."
However, inspectors did find that in recent months the pace of change to improve and develop provision and services for children and young people with SEND has improved. This was found to have been driven by new senior leaders in the SEN and disabilities team, along with new initiatives that have been introduced by frontline officers.
Meanwhile, the leadership of the clinical commissioning group was found to have been strengthened, with the work of the designated medical officer and designated clinical officer, combined with improved joint commissioning arrangements, contributing to improvements to support for children with SEND.