Mark Lever, the National Autistic Society's chief executive, said that genetics played a part in autism but it was not inherited in a straightforward way

The siblings of autistic children are nearly twice as likely to develop the disorder than was previously believed, research suggests.

A study published today tracked the development of 664 infants with an autistic older sibling until the age of three, at which point they were tested for the condition.

Almost 19 per cent of participants developed autism spectrum disorders, considerably higher than previous estimates which pegged the risk for siblings at between 3 and 10 per cent. "This is the largest study of the siblings of children with autism ever conducted," said the lead author, Sally Ozonoff, a professor of psychiatry and behavioural sciences at the University of California. "There is no previous study that identified a risk of recurrence that is this high." The risk was higher for boys, with 26 per cent of brothers of autistic children developing the disorder, and greater still for children with more than one autistic older sibling, 32 per cent of whom were diagnosed. Four-fifths of autistic children are male.

Estimates of the prevalence of autism have risen dramatically over time, from four or five in every 10,000 a generation ago to a generally accepted rate of one in 110 today. But the exact causes of the developmental disability remain a mystery.

The research, conducted at 12 sites across the US and Canada and published in the American journal Pediatrics, adds weight to the prevailing view that genetic factors play a critical role in the development of autism.

Professor Ozonoff said the findings highlighted the need for close monitoring of the infant siblings of autistic children. "This may require more than the normal surveillance that a paediatrician might typically do," she said.

She added: "Parents often ask what is the risk of having another child with autism and, until now, we were really not sure of the answer."

But she stressed that the estimates were averaged across all families, meaning that "for some families, the risk will be greater than 18 per cent, and for other families it would be less".

Mark Lever, chief executive of the National Autistic Society, said he welcomed the study but that it did not mean all parents of autistic children had an 18 per cent 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," he said.

"If any families already have a second child who they suspect might have autism we would advise them to seek professional diagnosis as soon as possible."