The finding which amazed even the scientists conducting the study provides one of the first clues as to a possible cause of the condition.

The study, by scientists in Sweden, Denmark and the United States, stumbled across the connection almost by accident.

It is being taken seriously because autism has long been thought to result from environmental factors.

Between 133,000 and 200,000 British children are thought to be autistic, but nobody knows for sure, or whether their numbers are increasing, because they are not counted.

But the numbers of babies born with the condition in California has risen more than seven times in the past two decades, convincing scientists that pollution must be to blame.

The new research, which traced nearly 5000 Swedish children from infancy to at least six years old, set out to investigate links between air pollution and asthma and other allergies.