Some patients are waiting over 19 weeks for an autism assessment, according to government statistics.
NHS data shows that patients at 10 out of 25 English health trusts waited an average of 137 days or more following referral, against a target of 91 days, in spring 2018.
The National Autistic Society (NAS) said the limited NHS statistics were "disappointing" as they only cover about a quarter of people referred.
The society has been campaigning for a trust by trust breakdown of waiting times but said these initial figures do not provide a clear national picture.
Jane Harris, director of external affairs and social change at the NAS, said: "Long waits can be traumatic for autistic children, adults and their families, who are often already vulnerable. We’re shocked that so few NHS trusts have submitted this information to NHS Digital. All trusts should take seriously their responsibilities to diagnose autistic people and report their waiting times publicly.”
She added: “We’ve been calling for national autism diagnosis data for years, because that’s how we can make sure that services are meeting the needs of autistic people and their families. But what’s been published isn’t enough to give us the national picture at all.”
“Whoever is in Government after the election needs to tackle the autism diagnosis crisis once and for all. That starts with a firm commitment to get this data in place.”