Autism is almost entirely genetic in origin, new research has suggested, with between 74 and 98 per cent of cases down to biological make-up.

A study conducted by the Medical Research Council looked at 516 twins, and found that rates of Autism Spectrum Disorder (ASD) were higher in identical twins who share the same DNA.

This means that the condition is far more heritable than previously thought. The study, which appears in the journal JAMA Psychiatry, also found that genes were responsible for autistic traits and behaviours in the general population.

Lead author Beata Tick said: "Our main finding was that the heritability of ASD was high. These results further demonstrate the importance of genetic effects on ASD, despite the dramatic increase in prevalence of the disorder over the last 20 years.

"They also confirm that genetic factors lead to a variety of autistic skills and behaviours across the general population."

But the researchers, who analysed data from the population-based Twins Early Development Study (Teds) - involving pairs of twins raised in the same household, by the same parents - said that they could not rule out the impact of environmental factors completely.

Co-author Professor Patrick Bolton said: "The comparison of identical and non-identical twins is a well-established way of clarifying the extent of genetic and environmental influences in autism.

"The novel aspect of this study was the inclusion of twins regardless of whether they had a clinical diagnosis. This enabled us to get a more accurate picture of how influential a child's environmental experiences and their genetic make-up is on ASD, as well as on subtler expressions of autistic skills and behaviours.

"Our findings add weight to the view that ASD represents the extreme manifestation of autistic skills and behaviours seen in the general population."

Researcher Dr Francesca Happe said that higher rates of autism in recent years could be put down to more correct diagnoses. The disorder, a spectrum of conditions which vary widely from person to person, may have previously been classed as a learning disability, and not recognised as autism, the BBC reported.

"Our findings suggest environmental factors are smaller, which is important because some parents are concerned whether things like high pollution might be causing autism," she said.

"Some people think there might be a big environmental component because autism has become more common in recent years but that's happened too fast for genetics to be a probable cause.

"The main consensus now is that the rise in diagnosis has more to do with increased awareness of the condition."

According to the website Autism Speaks, environmental factors which could be linked to autism include prenatal exposure to toxins, such as the chemicals thalidomide and valproic acid.