New data suggests some patients thought to have autism waited over 19 weeks for their first mental health appointments.
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.
NHS trusts said services had experienced "unprecedented" demand.
Jane Harris from the NAS said: "Long waits can be traumatic for autistic children, adults and their families, who are often already vulnerable."
"For the first time - after years of campaigning - we have NHS statistics on how long children and adults are waiting for an autism assessment in England," she said. "It's an important first step but the data itself is deeply disappointing."
The NAS has been campaigning for a trust by trust breakdown of waiting times but said the first such figures to do that do not provide a clear national picture.
'Can't wait any longer'
More than two years after a teacher suggested her son may have the disability, Natasha Webb, had to pay privately for her son to be assessed for autism.
The 47-year-old from Shrewsbury said she waited about three months for a first appointment via the NHS and although several reports were written by teams at Midlands Partnership NHS Foundation Trust, no official diagnosis was forthcoming.
Ms Webb, who is a full-time carer to 11-year-old Maxim, said it was an "exhausting, constant battle" to get him any help or support which left her feeling "written off".
As Maxim began secondary school, Mrs Webb said she borrowed money from her parents to have a private assessment, which cost £1,000.
"I couldn't wait any longer," she said. "I was up against educational welfare officers when he won't go to school and because we didn't have a diagnosis, it was my word against theirs."
NHS Shropshire Clinical Commissioning Group and Midlands Partnership NHS Foundation Trust said it continued to see "an unprecedented level of demand" for its autism services.
"We understand and appreciate the frustration felt by families who have been waiting longer than they should for an autism assessment," a spokeswoman said.
"Additional capacity has been put in place to address the current waiting list in Shropshire and Telford & Wrekin, prioritising those who have been waiting longest to receive an assessment," she added.
Figures were published for 39 NHS trusts, but average waiting times were suppressed for 14 of them due to low numbers and to protect patient confidentiality.
Of the remaining 25 trusts, 10 reported average waiting time 50% or more above the three month target between April and June 2018.
The average waiting times relate to only a quarter of patients - about 1,430 out of a total of almost 5,200, because in the majority of cases NHS trusts did not provide the dates on which patients were in contact with mental health services.
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Southern Health Trust reported an average waiting time of 221 days between April and June 2018.
Rob Guile, from the trust, said demand for its services had grown by more than 20% since 2015.
"Whilst we have received a slight increase in funding during this time, the increased awareness of autism and subsequent demand has not resulted in as big a reduction in our waiting lists as we'd hoped," he said.
"We agree that the current waiting times are not acceptable for local people," he added, saying the trust was in discussions to review its service and funding.
Norfolk and Suffolk NHS Foundation Trust reported an average wait of about 196 days over the same period.
Stuart Richardson, chief operating officer of Norfolk and Suffolk NHS Foundation Trust, said: "The average number of monthly referrals received by the Suffolk Youth Autism Diagnostic Service has increased by 37% from 19.2 in 2017 to 26.3 so far in 2019.
"We are working alongside our commissioners to reduce waiting times and to review the current service."