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If a child has autism, the risk of a younger sibling also developing the disorder is higher than previously thought, says a large US study.

Writing in Pediatrics, researchers from the University of California Davis studied 664 infants up to the age of three.

They found that the average risk was 18%, not 3-10% as estimated before.

An autism charity says genetics only play a part in the disorder.

Autism is a complex disorder which affects a child's ability to think, communicate, interact and learn.

The study involved infants from 20 different places across the US and Canada, who were six to eight months old at the start of the research. They all had older siblings with autism.

The researchers, from the Mind Institute in California, followed the children's development up to the age of three years, when they tested for autism.

Of the 664 participants, a total of 132 children were found to meet the criteria for an autistic spectrum disorder. Of these, 54 received a diagnosis of autistic disorder and 78 were considered to have a milder form of autism.

All the children were tested using an autism diagnostic tool, which measures non-verbal cognitive, language and motor skills.

Among the study participants, 26.2% of male infants compared to 9% of female infants were diagnosed with a form of autism.

Previous studies have shown that autism is more common in males than females - 80% of all affected children are male.

In families with more than one older sibling with autism in the study, the average risk was 32%.

Prof Sally Ozonoff, lead study author and professor of psychiatry and behavioural sciences at the Mind Institute, University of California, explained the higher average risk figures in her study.

"Previous studies used different diagnostic criteria. This is the largest study of the siblings of children with autism ever conducted, the first to follow families up to the time of diagnosis as opposed to looking back once they have been diagnosed."

But she was careful to emphasis that the figures were estimates averaged across all of the families.

"So for some families the risk will be greater than 18%, and for other families it will be less than 18%. At present we do not know how to estimate an individual family's actual risk.

"Genes is a large part of autism, but it's not the whole story. Non-genetic factors are also important, but we don't know exactly what they are."

The National Autistic Society (NAS) says there are over half a million people in the UK with autism - that is around one in 100.

Mark Lever, chief executive of NAS, said the society welcomed the research.

"However, we would like to stress that this study does not mean that all parents of children with autism have an 18% chance of having another child with the condition.

"While genetics are thought to play a part in autism, the condition is not inherited in a straightforward way. Parents of multiple siblings with autism may, however, be at an increased risk of having subsequent children with the condition than those with just one child.

"It is essential that healthcare professionals also take heed of these findings and work proactively with families to monitor the development of siblings of children with autism."