Circumcision before the age of five may double a boy’s risk of developing autism, controversial research suggests.

Scientists believe the finding may be linked to stress caused by the pain of the procedure.

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The study of more than 340,000 boys in Denmark also found a 46 per cent higher rate of autism spectrum disorder (ASD) diagnosis amongst boys circumcised before the age of 10.

The finding was a correlation, which means autism diagnosis rates seem to increase amongst circumcised boys, but the study did not prove that the medical procedure was in fact the cause of the increase.

The finding has been criticised as being “far from convincing”.

Professor David Katz, from University College London, who chairs Milah UK, a body that speaks for the Jewish community on issues related to circumcision, said: “This report is far from convincing [because] correlation does not equal causation.

Professor Katz suggested that the link between autism and circumcision may be explained by genetic factors amongst Jewish and Muslim populations, and not the act circumcision itself.

While it is considered unacceptable today to circumcise boys without proper pain relief, it is not possible to make the procedure completely pain-free.

The pain of circumcision is likely to be more severe in very young babies both during and after the operation.

Painful experiences in newborns have been shown in both animal and human studies to be associated with long-term alterations in pain perception, a characteristic often seen in autistic children.

Professor Morten Frisch, of the Statens Serum Institut, Copenhagen, who led the research, said the mechanism by which stress and pain in early life may increase the risk of brain problems later in life is “incompletely conceptualised”.

The Professor said his study should prompt further research, but did not claim it as conclusive proof of a link between autism and circumcision.

The findings are reported in the Journal of the Royal Society of Medicine.