Living near a busy road could be linked to a dramatic increase in the risk of childhood autism.

Early exposure to traffic pollution, either in the womb or during the first year of life, more than doubled a child's chances of having the disorder, scientists in California found.

Children from homes with the highest air pollution levels were three times more at risk than those from the least exposed homes.

Experts called the finding "important" but said it did not prove a causal link between pollutant chemicals and impaired brain development.