More than a quarter of English councils are acting unlawfully by discriminating against children with autism, according to a report by legal experts.
Forty-one of 149 local authorities have policies that denied families social care assessments, the Disability Law Service said.
Sir Ed Davey, the acting Liberal Democrat leader and a patron of the charity, which provides free legal advice for disabled people, said the way some families of autistic children were being treated was "unlawful, discriminatory and unacceptable".
In an open letter to councils highlighting "systemic and widespread discrimination against autistic children", he said: "These children have only one crack at childhood, and if the law and the local authorities don't support them then those precious moments will be wasted.
"It's vital those in charge of social care in every local authority step up to the plate, review their policies and make sure children are getting the social care needs assessments and support the law says they should."
Children with autism have the right to be treated the same as other disabled children under the Equality Act, and local authorities are bound by law to assess disabled children and deliver the services they need. Such support can include various clubs and care services.
However, the charity's report found councils were blocking assessments and would only allow them if the child had an additional disability, showed a level of challenging behaviour or had been given an official diagnosis of autism.
Jane Harris, the director of external affairs at the National Autistic Society, said: "This is a damning report, revealing how autistic children are routinely being failed by a social care system starved of investment.
"Parents tell us they're asking for help in desperate situations, but too often they're being turned away.
"It is completely unacceptable for a council to deny a child an assessment simply because an autistic child does not have another condition. It is equally wrong for a council to withhold support from someone just because they do not have a formal diagnosis.
"Families are left struggling to support their children without training or funding, and without extra help, such as short breaks, they can end up isolated and alone, struggling to cope day to day.
"Councils should make decisions about support based on a child's needs. Without this, autistic children will struggle at home and at school.
"No one would accept this situation for their child, and we won't accept it for autistic children. The Government must make sure councils follow the law and have funding to support autistic children properly."
The Government said it was increasing high-needs funding for local authorities by £780 million this year and a further £730 million in 2021-22, boosting the total budget for supporting those with the most complex needs to more than £8 billion that year.
The Local Government Association (LGA) said councils were "doing all they can". Cllr Judith Blake, who chairs the LGA's Children and Young People Board, said: "Councils and their partners are working hard to try and support children with all forms of special educational needs and their families.
"However, councils are under huge strain as a result of the rising demand for support – seeing an increase of 10 per cent in Education, Health and Care Plans in the last 12 months alone.
"It was good the Government acknowledged this with additional funding for council high needs budgets for 2020-21."