The team compared the behaviour of pairs of identical twins, who share all their DNA, and non-identical twins, who have only half their genes in common.

They found the condition was far more common in identical twins than in non-identical ones. The study, in the journal JAMA Psychiatry, concluded that DNA was behind between 74 to 98 per cent of autism cases.

If genes are key, the recent rise in cases could be explained by more people being correctly diagnosed with the condition.