The warning has come from the National Autistic Society (NAS), which this week has called on NHS England and the government to take urgent action.
The NAS says that without a diagnosis, people on the spectrum are often left without support for years and are at serious risk of developing mental health problems such as anxiety and depression.
This has a knock-on impact on families, who have to deal the additional strain of looking after their loved ones without help. The situation also has clear implications for schools.
The charity has raised concerns with the chief executive of NHS England Simon Stevens and health secretary Jeremy Hunt in letters signed by more than 11,500 people, including prominent autism experts Professor Simon Baron-Cohen and Dr Judith Gould.
The letters call on NHS England to act by recording and publishing key data on local NHS performance and adopting a standard so that no-one waits longer than three months for a diagnostic appointment.
Guidance from the National Institute of Health and Care Excellence (NICE) recommends a maximum three months between referral for a diagnosis and a first appointment. However, research from City University London and Goldsmiths University shows that this is not being met.
Parents who responded to a survey about their experiences of attaining a diagnosis for their child reported that the average delay between first contacting a healthcare professional and receiving a diagnosis was 3.6 years.
Mark Lever, NAS chief executive, 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.
"Reducing waiting times will drastically improve the lives of hundreds of thousands of people on the autism spectrum and their families, and it can also save money at a time when public funds are strained."