"This is a dramatic increase in the number of kids classified as autistic or documented on the spectrum of similar disorders," said Dr Beverly Mulvihill, University of Alabama at Birmingham (UAB) associate professor of public health and co-author on the study.

"It is not entirely clear what is causing the rise, but we know major collaborative efforts are needed to improve the understanding and lives of people and families impacted."

Data for the study was provided by the Autism and Developmental Disabilities Monitoring Network.

The group collects information on new diagnoses of 'autism spectrum disorder' (ASD) from 11 partner organisations in Alabama, Arizona, Colorado, Florida, Georgia, Maryland, Missouri, North Carolina, Pennsylvania,

ASD is an term used to describe three distinct types of condition that can be labelled as 'autism'; classic autism, Asperger's syndrome and atypical autism.

The study also found that gender seems to be a factor in autism -- boys are 4.5 times more likely than girls to have ASD, a result also found in previous studies.

"It still is not clear why males more frequently are affected," said Dr Martha Wingate, UAB assistant professor of public health and study co-author.

"One thing we know for sure is that more research is needed to quantify the effects of single or multiple factors such as diagnosis patterns, inclusion of milder cases and other components."

But not all experts in the field are convinced that there is a significant rise in ASD diagnoses amongst children.

Dr Max Wiznitzer, paedriatic neurologist at Rainbow Babies & Children's Hospital in Cleveland, Ohio, believes that many children are being incorrectly labelled as autistic.

He points out that 54% of children in the study were diagnosed with ASD, meaning that over half did not meet the criteria.

"This suggests that over-diagnosis is occurring in the population," Dr Wiznitzer said.

The report contained evidence from parents who suspected autism in their children before the age of two.

Parents found that they would have to wait up to three years before they could get an official diagnosis.

"That is...too late," said Dr Wiznitzer.

"We need to do a better job of identifying children earlier on."