More than two thirds of local authorities in England are unaware of how many people with autism live in their area, according to a report.
The National Autistic Society (NAS) is using its 'I Exist' report to call for a wider study into the number of people with autism in the UK.
The study also found that 63% of adults with autism in England do not get enough support to meet their needs.
The NAS says it wants the government to "think, act, and transform lives".
The charity said delivering services was being made difficult by the lack of official records of people with the developmental disability.
The report also found 65% of local authorities did not know how many autistic adults they were supporting.
It also found that at least one in three adults with autism experiences severe mental health difficulties due to a lack of support.
'Remove barriers'
Some 60% of parents who participated in the survey said a lack of support for children with autism would lead to them having a greater need for help later on in their lives.
Amanda Batten, NAS head of policy and campaigns, said: "For too long adults with autism have found themselves isolated and ignored.
"They struggle to access support and are often dependent on their families.
"It does not have to be like this. 'I Exist' is the message from adults with autism who want their needs understood and the barriers to support removed.
"The right help at the right time can have a profound effect. We are calling on the government to think, act, and transform lives."
More than 1,700 people took part in the survey, who were adults with autism and their families.
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