Children with autism are having to wait an average of three-and-a-half years before diagnosis, experts claim.
Adults with the condition have to wait an average of two years, causing anxiety and depression, the National Autistic Society and academics said.
In a letter to NHS England, 12,000 of the society's supporters have demanded action to cut the wait for diagnosis.
NHS England said it was working to cut waiting times, but diagnosis could be complex and involve different agencies.
In a letter to The Times, a group of six experts and campaigners said some families were breaking down under the strain of providing care without support.
They called for "national action to reduce diagnosis waiting times and stop autistic people from suffering avoidable mental health problems".
This could save the National Health Service an estimated £67m per year, they claimed.
"If the NHS is going to help people on the autism spectrum and make the savings it needs to, it must prevent mental ill-health," they write.
"Improving autism diagnosis is an opportunity to do this."
Signatories of the letter to the Times include Mark Lever, chief executive of the National Autistic Society; actress Jane Asher, the society's president; Cheryl Gillan MP, chair of the all-party parliamentary group on autism; and Prof Simon Baron-Cohen, director of the University of Cambridge Autism Research Centre.