As many as one in 175 primary school children may suffer from autism, 11 times higher than previous estimates.
The findings mean that the cost of education and care of sufferers could be £5 billion a year, but researchers said their figures were an underestimate, if anything". Dr Fiona Scott, a research co-ordinator at the Autism Research Centre at Cambridge University, said:
"We only included children who had a definite clinical diagnosis, so any child that had been statemented as autistic or with autistic spectrum disorders but not diagnosed clinically was not counted"
Previous studies have estimated that five in every 10,000 children aged between 5 and 11 were autistic, but the new study, of children in Cambridgeshire, puts the rate at 58 in 10,000.
Extrapolated across Britain, this suggests that 30,000 primary school children and tens of thousands in other age groups may have clinical autism, an incurable condition which will mean that they will need support for the rest of their lives.
Significantly, the study established that one in eight children with special education needs was suffering from some form of "autistic spectrum disorder".
Autism is a very contentious issue in medicine and education and the study was undertaken to establish basic figures for the number of primary school age children with the condition. It affects boys three times more frequently than girls, although it is not known why, and there is much debate among doctors on how the condition should be defined.
David Potter, of the National Autistic Society, called for a national register of children diagnosed with the condition, which affects their ability to socialise, communicate and learn. While local education authorities may "statement" a child's learning difficulties and special schooling needs, a true diagnosis of the child's underlying condition is often not recorded.
As a result, the Government, health authorities and local education authorities have had no firm foundation on which to base strategies for dealing with affected children. However, the 11-fold increase on earlier studies has enormous cost implications for the Government.
Last year, a report for the Mental Health Foundation, co-authored by Professor Martin Knapp from the Institute of Psychiatry, put the total economic cost of autism at a £1 billion a year, using the "textbook" rate of five in 10,000 children. Using the Cambridge figures, experts conceded last night that the true cost, allowing for less badly affected children, could be £5 billion a year.
Tony Charman, a senior lecturer in clinical psychology at the Institute of Child Health who is about to begin a study in the South-East, said: "If it is the case that the prevalence is higher by a factor of several fold that obviously has important and expensive implications for pre-school and specialised health services."
Professor Knapp's report put the cost of autism at nearly £3 million over a lifetime for a severely affected autistic child with learning difficulties, nearly £800,000 for people with "high functioning" autism and more than £500,000 for people with Asperger's syndrome.