As many as 80% of doctors need extra training to help identify and manage those with autism more effectively, a study has shown.
And services set up to deal with the condition could be failing half of the 400,000 adults in England with autism, according to the National Audit Office (NAO).
It says that many people with autism do not have a learning disability, but services are mostly set up for people with illness, mental health problems, and physical or learning disabilities.
Of the 1,000 GPs surveyed by the NAO, four in five thought they needed additional guidance concerning the condition, while many admitted not knowing where to send people with autism due to a lack of specific services.
The study - called Supporting People with Autism through Adulthood - said: "In our survey of GPs, 64% told us that they referred adults with suspected high-functioning autism to adult mental health services, and 19% to learning disability services.
"(A total of) 12% were not sure where they should refer such patients, and only 10% reported that they would refer them to a specialised autism diagnostic service."
The NAO called for change in the form of specialist support services after finding that most NHS organisations and local authorities did not even know how many people with autism lived in their area.