One in 100 adults has autism, and the condition is six times more common in men than women, a major report has revealed.
The study, by NHS Information Centre, found that one in 50 men has autism, compared to one in 300 women. It was also revealed that one in three adults with learning difficulties is autistic.
The findings suggest the condition is far more widespread among adults than previously thought.
Previous studies of autism have largely focused on children and until recently little has been known about the prevalence in adults.
Researchers asked over 7,400 men and women a series of questions to detect signs of autism.
Those whose answers indicated they may have the condition went on to take part in a further test.
In a separate survey, researchers interviewed more than 300 adults with learning difficulties and their carers.
The combined results of the two studies found that 1.1% of all adults had some form of autism - figures similar to the autism rates among children.
The study has been welcomed by leading charity, The National Autistic Society (NAS), which has campaigned for years for recognition and better understanding of adults with autism.
Mark Lever, chief executive of NAS, said: "This study shows that autism has been with us for a long time and that a large group of adults has been consistently overlooked by services and society.
"There has long been a tendency to view autism as solely a condition affecting children but this is the first study to find that the prevalence of autism is roughly the same for adults as it is for children.
"Many people with autism currently face a battle to get appropriate support, with 63% of adults saying they do not have enough to meet their needs. Now that we know how many adults with autism there are in England local authorities should be better able to estimate local need and plan services accordingly."
A recent study found that one in three children who had been previously diagnosed with autism no longer had the diagnosis when their parents were later questioned, raising the question of whether a child could 'grow out of' autism.
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