Local authorities spent £4.3m on handling and contesting special educational needs tribunals last year - an increase of nearly a quarter compared to the year before.
A freedom of information request by Ambitious about Autism found local authorities spent £4.3m in 2017/18, an increase of £846,000 on the previous year.
Research by the charity also found 95% of tribunal decisions were either won by parents, or withdrawn or conceded by local authorities.
Jolanta Lasota, chief executive of Ambitious about Autism, said: 'The fact that the majority of special educational needs cases are being won by parents or conceded by local authorities is a clear sign that initial decisions about educational support are not being made in the best interests of the young person.
'However, the lengthy legal process to correct these mistakes means children are waiting far too long to get the support they need to thrive in school. This delay has damaging consequences on children’s happiness and ability to thrive academically.'