Inspectors found that health professionals in Merton lack training in helping young people through education, health and care (EHC) plans, including assessing, planning and reviewing their support.
The joint inspection by Ofsted and the Care Quality Commission was also critical of the ineffective partnership between education and care services provided by Merton Council and health support through NHS Merton's clinical commission group (CCG).
Leaders' strategic decisions have not ensured that the area is "far enough on" with implementing improvements in SEND support, states an inspectors' letter published this week.
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A failure to oversee the quality of EHC plans is a particular concern raised, in particular the poor contribution from health professionals.
The letter states: "EHC plans are typically overcomplicated and include an overwhelming range of aims. The health contribution to and inclusion in these plans is weak."
Inspectors say children's nurses, child and adolescent mental health service (CAMHS) workers and dietary specialists need to be more closely involved in the planning of SEND support.
This is vital to ensuring plans for children are accurate and up to date, say inspectors.
"When they do contribute to EHC assessments, health partners do not always receive copies of a draft EHC plan," adds the inspectors' letter.
"This means they have no opportunity to check for accuracy before plans are finalised.
"There is no effective process to amend or update EHC plans when they include inaccurate information. Therapists report outdated, incorrect information regarding a child's needs being ‘pulled through' from previous years, despite updates being submitted in an attempt to correct the information."
Due to the severity of their concerns, inspectors have ordered the council and CCG to produce a written statement of action on how they plan to improve SEND support.
However, work to ensure children are involved in decisions around their care is praised. "As a result, leaders know about children's and young people's ideas and views," the letter adds.
The "expertise" of early years workers in identifying SEND concerns among children is also praised. This is due to their strong training in conditions such as autistic spectrum disorder and language delay, which "helps providers to gather more compelling evidence of suspected needs at an earlier stage".
Health visitors' "persistence" in making contact with children in temporary housing is also welcomed by inspectors for identifying "potentially unmet needs".
A spokesperson for Merton Council and NHS Merton CCG said: "We take our role in caring for children with special educational needs extremely seriously and Ofsted has identified many strengths in the way we are currently working.
"We are taking immediate steps to address the concerns raised. Our written statement of action will set out in particular how health agencies and the council will work more closely together in assessment and care planning for children and young people with special educational needs."