Researchers have found the ailment is really a combination of two separate illnesses, each controlled by different sets of genes.

The discovery, outlined yesterday at the British Psychological Society conference, is expected to cause intense interest among psychologists. Many believe prospects of uncovering the condition's genetic causes and finding new treatments have been significantly boosted.

'In effect, we're saying there is no such thing as autism, but two separate conditions which - if they occur at the same time in the same child - give rise to symptoms that we associate with autistic individuals,' said Professor Robert Plomin, of the Institute of Psychiatry, London. 'That has tremendous implications for helping these children.'

Autism sufferers display rigid, obsessive behaviour and largely lack an ability to relate to family members. More worryingly, numbers of cases have been rising rapidly in recent years - with more than 2,000 children being diagnosed in Britain every year. Scientists are still unsure why this rise is occurring, however.

Typically sufferers - who start to display symptoms around the age of two or three - are compulsive, withdrawn and have explosive outbursts.

'Diagnoses depend on two observations,' said Dr Angelica Ronald, of the Institute of Psychiatry. 'First, the social component: autistic children do not understand that other people have minds of their own. They are tactless and uncommunicative. Second, there is the non-social aspect. Children are obsessive about objects and pre-occupied with details of places or events.'

In the past, psychologists assumed these two sets of symptoms had the same cause. But a major study led by Ronald and Plomin of 4,000 pairs of twins has found this to be incorrect. Autism's two sets of symptoms are actually acquired quite separately.

'The two sets of symptoms are associated with two completely different sets of genes,' Ronald said. 'Only when a person inherits extreme versions of both do they exhibit the symptoms of full autism.'