The "significant areas of weakness" detailed in a new inspection report has resulted in Ofsted ordering Peterborough Council and the Cambridge and Peterborough Clinical Commissioning Group to submit a joint written statement of action on how they will improve local practice.
However, the council said it has insufficient funding to deliver the reforms at the pace expected.
The inspectorate found that many parents and carers in Peterborough felt the 2014 reforms had made little difference to outcomes or provision, with inspectors concluding that parental concerns over the quality of education, health and care plans, and delays to access health services were well founded.
While pathways for some services are well established, too many children and young people with SEND wait too long to get the help they need. Some parents said they had been repeatedly told by professionals in education and health services to "wait and see" if things get better.
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Meanwhile, the quality and range of provision for 18- to 25-year-olds lags behind that for younger children and young people, which is affecting their aspirations to live full and meaningful lives.
Joint planning, including commissioning, and intervention are not sufficiently well established to ensure that all agencies and services play an active role in meeting the requirements of the 2014 reforms.
Ofsted said outcomes for children and young people are not sufficiently targeted, measured or evaluated to inform leaders about the impact of the work to implement the reforms effectively.
It also said that the role of the designated clinical officer is not developed enough to support the health requirements of the 2014 SEND reforms.
Ofsted acknowledged that progress on implementing the reforms has gained momentum since 2016, but added there was still a lot of work needed to ensure the reforms are implemented fully and effectively.
Council leaders are aware of the problems and actions to address them are often in place - although the inspectorate concluded "too many" of the actions are at an early stage of implementation or effectiveness.
They also stated that "leaders are not in agreement that the weaknesses are significant enough to require a written statement of action".
Councillor Lynne Ayres, cabinet member for children's services, education, skills and the university for the city council, called for more money for SEND provision.
"Making the changes that we know we need to, with the funding that we have available, is undoubtedly challenging," she said. "We simply don't receive the funding we need from the government to provide SEND services and we are not alone in this - many other councils are painting a similar picture.
"That said, we accept the findings and the council, health services and our partners will do everything we can within the available resources to make the required improvements."
Council Leader John Holdich and executive director for people and communities Wendi Ogle-Welbourn have signed a letter sent to the government by charity Family Voice raising concerns about current funding levels for SEND services and calling for additional investment.