A council has been told to reconsider all recent decisions on the delay of school places for summer-born children.
Parents of children born between April and August can apply for their child to start reception class a year later.
But the Local Government Ombudsman (LGO) said Warwickshire County Council failed to properly consider Helen Leavitt's application to delay her son's start date.
A council spokesman said it was "currently reviewing policy".
The LGO found the authority's information was "incorrect or misleading".
Mrs Leavitt and her husband had applied for their son to start reception at the age of five because they felt he was not ready for full-time schooling - a judgement supported by nursery staff and three head teachers.
But Warwickshire County Council said it was not in her son's "best interests" for him to be educated outside of his peer group and he would have to start in year one.
She complained to the LGO, which said the council failed to take into account the parents' and head teachers' views and did not follow the school admissions code.
It has ordered the local authority to apologise.
It must also review applications it has received for summer-born children to start in reception in 2020 and 2021.
Following the ruling, Mrs Leavitt said: "We all recognise how important reception year can be and the thought that my child could miss out on that, it's heartbreaking."
A council spokesman said it was "currently reviewing policy" and guidance to parents and staff.
The LGO report comes after the Office of the Schools Adjudicator also found the council's arrangements for summer-born children and others applying for a later start "do not conform with the requirements" in the school admissions code.
It said this meant guidance was "unlawful" at the time "thousands" of parents were applying for school places.
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