The National Autistic Society said “huge” numbers of people are waiting to be seen across England, with some potentially having to wait years for a diagnosis.
Referrals can be made by GPs, health professionals, speech therapists and school special education needs staff.
Nationally, 88,000 people were waiting for an assessment in December – up from 64,000 in December 2020.
Tim Nicholls at the National Autistic Society, said the figures are "huge" and is worried people will have to wait months or even years for a diagnosis.
Mr Nicholls urged the Government to invest in rolling out support and diagnosis services, as outlined in its five-year autism strategy.
In a joint policy paper in July, the Department for Education and Department of Health and Social Care vowed to provide high-quality and timely autistic diagnoses.
NICE guidance states an early and quick diagnosis is vital to autism treatment, and that all providers should aim to provide patients with their first appointment within 13 weeks of being referred.
The number of patients receiving their first appointment before the 13-week target in Doncaster rose in 2021, when roughly 1,250 people were seen within that time, up from 1,160 the year prior.
NHS Digital did not provide figures on patients waiting more than 13 weeks.
"These figures are still new and don’t give us an accurate picture of just how long people are waiting for a diagnosis across England," Mr Nicholls added.
"The NHS needs to work to make this data more robust, so areas can be held to account for carrying out a diagnosis in good time."
A Department for Health and Social Care spokesperson said: "We know how vital it is to reduce the time taken for people to receive an assessment of autism and to ensure people get a timely diagnosis.
"To better understand autism diagnosis waiting times across the country NHS Digital has changed the way this data is collected, and we expect this to start providing a much clearer picture going forward."