Delays in autism diagnosis are pushing families to crisis point, a charity has warned.
The National Autistic Society (NAS) says families and individuals are suffering from anxiety and depression because of years spent waiting to be diagnosed.
Average waiting times of more than three-and-half years for children and two years for autistic adults are well above Nice guidelines, which state that there should be a maximum of three months between referral and diagnosis.
City University London and Goldsmiths University of London asked parents about their experiences of attaining a diagnosis for their child to calculate the average delay between contacting a healthcare professional and receiving a diagnosis.
Their research also found that 84% of parents who responded to the survey found the process stressful and more than half said they were dissatisfied.
NAS chief executive Mark Lever said: "Too many families and individuals are being pushed into anxiety or depression by years waiting for an autism diagnosis.
"It is deeply traumatic not to know why you or your child feel or act differently to those around you.
"Simply put, a diagnosis is life-changing and essential to getting support and services."
Early diagnosis could also save the NHS millions of pounds by reducing the number of GP appointments, emergency admissions and use of mental health services.
The National Audit Office believes identifying and supporting eight per cent of adults with high-functioning autism and Asperger syndrome - a form of autism - would save £67 million per year.
The NAS has sent a letter signed by 11,627 people, including autism experts Professor Simon Baron-Cohen and Dr Judith Gould, to health secretary Jeremy Hunt raising its concerns.
It calls on NHS England to record and publish key data on local NHS performance and to cut waiting times for a diagnostic appointment to below three months.
They also want the government to write timely access to autism diagnosis into its NHS England mandate.
An NHS England spokesman said: "Diagnosis of autism spectrum disorders should, of course, happen as soon as possible, but often is complex and involves many different professionals and agencies."
Autism, which affects how people communicate with and relate to other people, is a spectrum condition, meaning that while all those diagnosed with the disability share certain difficulties, the condition will affect them differently.