The international study suggests the older fathers are more likely to have grandchildren with autism than their younger counterparts.

  The mechanism is not clear but it is thought they transmit the "silent mutation" to their grandchildren.

  But the experts have urged cations, stressing that the autism is the result of many factors.

  The study, looking at almost 6000 people with this condition, is published in the journal Jama Psychiatry.

  According to the National Autistic Society, more than one in every 100 people in UK may have this condition.

  Previous studies suggested that older fathers may be at greater risk of having children with autism than the younger dads.

  But the team of UK, Swedish and Australian researchers said it is one of the first pieces of evidences to show the risk can be passed on - rather than just straight to future generations.

  The "silent mutations", - changes in genetic material - are likely to have no obvious impact on the older father's own children, but they may build up through subsequent generation or interact with other genes and environmental factors, to increase the chance of their children developing the condition, the researchers said.