Health reporter, BBC News
Adults with autism can become socially isolated
Thousands of people with autism in England are failing to get the diagnosis and specialist help they need, the National Audit Office says.
Most local authorities do not know how many adults with autism live in the area and provide no specific services.
Yet better support in areas such as housing and employment could save the taxpayer millions of pounds in the long run, a report concluded.
The government is due to publish its first autism strategy later this year.
People with autism struggle to communicate socially and have trouble understanding facial expressions and tone of voice and recognising emotions.
Around half of those with autism also have a learning disability, but for those who do not - those with high functioning autism, such as Asperger Syndrome - accessing support for housing, further education and employment can be particularly hard, the NAO said.
Three-quarters of local authorities have no commissioning plan in place for specialist autism services and 65% struggle to find appropriate housing.
The report also found that children with autism are often abandoned by specialist services when they turn 18, due to a lack of adult services or a proper transition plan.
An average GP is thought to see two adults with undiagnosed high-functioning autism every six months.
But the vast majority say they need better training to identify and manage patients better.
And although there are education programmes now in place, there is still a lack of expertise at job centres, with only 200 of 500 disability advisors trained to help people with autism, the NAO said.
If local authorities identified just 4% of people with high-functioning autism, and offered them specialist support with living independently or getting a job, the cost would be cancelled out by savings in other areas, the NAO calculated.
Diagnosis and support for 8% of that population would actually save £67m a year, it added.
Mark Davies, NAO director of health value for money studies, said there would be people in their 40s and 50s who had never been diagnosed with the condition.
"We would like people to look at our report and the modelling we have done because we think there is a good case for having more specialist support."
Care services minister Phil Hope said the first ever autism strategy would be published at the end of 2009 and a study was being done to work out exactly how many adults have autism in England.
"Our commitment to do this will have the force of law - in the first ever Autism Bill going through parliament."
AUTISM IN ENGLAND
An estimated 400,000 adults have autism
Half of those do not have a learning disability
Only 15% are in full-time employment
Geoffrey Maddrell, chairman of Research Autism, agreed there was an "appalling" lack of joined up and accessible provision for adults with autism.
"With the correct employment support and mentoring, many of these adults can sustain long-term education and career paths in various sectors.
"But at present this is not happening in many places."
Mark Lever, chief executive at the National Autistic Society, said the government could not ignore the "huge cost savings and benefits", identified by the NAO.
"Neither the government, people with autism nor the taxpayer are getting value for money from existing autism services and support, leaving those affected by the condition feeling isolated, ignored and often at breaking point."
Professor Simon Baron-Cohen, an expert in autism at the University of Cambridge, said even when people were diagnosed they were often left "isolated, unemployed, lonely, and at risk of developing potentially preventable secondary depression".
"The hope is that this report will lead to a step change in how we meet the needs of this invisible group in our society."