Ministers rejected plans for a new special school in an area that Ofsted later exposed as having “significant” numbers of children forced into home education because of a places shortage.
Inspectors said Hertfordshire had “widespread and/or systemic failings leading to significant concerns” about children with special educational needs and disabilities (SEND).
Ofsted and the Care Quality Commission (CQC) said leaders had “not acted with the necessary urgency to address longstanding, systemic and significant weaknesses”.
It is the fifth council to get the lowest possible rating since new SEND inspections were introduced in January.
