Researchers who surveyed children aged five to nine years in the Cambridgeshire area say the findings should help education, health and social services prepare for the future.
Their figures confirm that one percent of primary school aged children currently have an existing diagnosis of autism, but also suggest that there may be many more currently undiagnosed and in need of support.
The National Autistic Society has called for the research to be replicated more widely, to confirm the figures on a national level.
The research team, led by Professor Simon Baron-Cohen at the Autism Research Centre at Cambridge University, used three different methods to estimate the prevalence of autism-spectrum conditions in children attending 79 schools in and around Cambridge.
They surveyed children using the Special Educational Needs registers, then surveyed parents of 11,7000 children in the area. Of the 3,373 completed surveys, 41 cases of autism spectrum conditions were confirmed. Finally, the same parents were asked to complete a Childhood Autism Screening Test, to help identify any undiagnosed cases - this found an additional 11 children.
Researchers calculated that, if the figures were extrapolated to the wider population, for every three cases that are known, there may be a further two undiagnosed cases.
While figures for autism are significantly higher than 30 years ago, increases may be explained by improved awareness by both parents and professionals, increased diagnostic services, and a widening of the criteria to include related conditions like Asperger Syndrome, which did not enter the diagnostic manuals until 1994.
The causes of autism are unknown, although it affects four times as many boys as girls. Diagnosis is rarely detected before the age of three, although screening toddlers at 18 months can indicate those who are at risk for a social communication disorder.
Professor Baron-Cohen said the study, detailed in June's British Journal of Psychiatry, was particularly significant because it did not rely on a single source of information, but instead combined information from three different sources.
He said such studies were important to help society put plans in place for the future. ‘The relevant services, including education, health and social services, can plan adequate provision for all those children and adults who may need support.'
Welcoming the report, the National Autistic Society said an accurate figure for the number of people with autism across the UK was vital to ensure that there is a sufficient level of services and support in place to meet their needs.
NAS chief executive Mark Lever said, ‘This is important research, which for the first time gives us an estimate of the number of people who don't have an autism diagnosis but may be in need of support. Getting the right support at the right time is vitally important and access to appropriate diagnostic services is crucial'
