Autism, and similar disorders, may affect up to one in 100 children, UK researchers have suggested.
Researchers warn autistic children will need special services
The figures suggest the condition is more common, as prior to the 1990s, experts said there were four to five cases per 10,000 people in Britain.
Researchers said it was unclear whether their higher estimate was due to better diagnosis or increased incidence.
Experts said the study showed services for people with autism spectrum disorders (ASDs) had to be improved.
Autism impairs social interaction, communication, and imagination. The spectrum also covers Asperger's syndrome.
Service boost needed
Researchers from Guy's and St Thomas' Hospital in south London, publishing their findings in the Lancet, looked at a group of 57,000 children aged nine and 10 in 2001.
They identified 255 who had already been diagnosed as having autistic disorders and 1,515 judged to be possible undetected cases.
A randomly selected sub-group of 255 children was chosen for in-depth clinical assessment.
The prevalence of "classic" childhood autism was 39 per 10,000, and that of other ASDs 77 per 10,000.
In total, autistic disorders affected 116 per 10,000 children.
The researchers extrapolated their findings to suggest one in 100 British children may have some form of autism.
Professor Gillian Baird, who led the research, said: "Prevalence of autism and related ASDs is substantially higher than previously recognised.
"Whether the increase is due to better ascertainment, broadening diagnostic criteria, or increased incidence is unclear.
"Services in health, education, and social care will need to recognise the needs of children with some form of ASD, who constitute 1% of the child population."
Autism 'no longer rare'
But experts said there was no evidence to link the increase in cases to the measles, mumps and rubella jab or thimerosal, a vaccine preservative which uses mercury.
An editorial in the Lancet by Dr Hiroshi Kurita of Tokyo's Zenkoku Ryoiku Sodan Centre says there has been a continued incidence of ASDs after MMR was withdrawn in a district of Yokohama City in Japan and in Denmark, where thimerosal was taken out of vaccines.
Professor Simon Baron Cohen, of the Autism Research Centre in Cambridge, said: "This new study establishes that autism spectrum conditions are no longer rare.
"Service planning is needed to adjust to these new prevalence rates, so that the education, health, and related systems can meet the needs of people on the autistic spectrum."
The National Autistic Society (NAS) said the study's findings fitted in with its own estimates of the incidence of ASD - and the need for improved services - but it was unclear why more cases were being seen.
The NAS said its helpline received 35,600 calls last year from individuals with autism and their families, many of whom were unable to access the services they require.
Mike Collins, NAS head of Education, said: "Current provision for those with the disability is deeply inadequate given the scale of the need.
"Government and local authorities must ensure that education, health and social services are adequately funded and all staff appropriately trained in order to meet the needs of those living with the disability and their families.
"Autism is a lifelong disability and when an individual's needs are not met the long term consequences both financially and for the individual's well being are profound."