The rise in autism since the 1980s could be partly explained by changes in how children are diagnosed, according to a new University of Oxford study.

Some youngsters diagnosed with severe language disorders in the 1980s and 1990s would today be diagnosed as having autism, it suggested.

Several experts and parents believe the rise is linked to environmental changes or other factors like reactions to the MMR jab.

Dr Andrew Wakefield, who believes the vaccine could be linked to bowel disease and autism, is currently appearing before the General Medical Council on charges related to his claims.

Today's study, published in the journal Developmental Medicine and Child Neurology, was led by Professor Dorothy Bishop, a Wellcome Trust principal research fellow at the University of Oxford.

A total of 38 adults (31 men and seven women aged 15 to 31) took part in the study and underwent interviews.

They had all been diagnosed with developmental language disorders when they were younger.

As part of the new research, their parents completed a revised autism diagnostic interview, which relies on symptoms when children are aged four or five.

The results showed that eight adults would now meet the criteria for autism and a further four would meet criteria for milder forms of autistic spectrum disorder.

The authors concluded: "Some children who would nowadays be regarded as unambiguous cases of autistic disorder were diagnosed with developmental language disorder in the past.

"This finding has implications for our understanding of epidemiology of autism."

In recent years, the criteria for diagnosing developmental language disorders and autism has changed, according to the researchers.

This has coincided with a marked rise in the rates of diagnosis of autism. According to the one study published in The Lancet medical journal in 2006, the figure until the 1990s was accepted as being about five people per 10,000.

Even using the narrowest definition of autism, this rose to almost 40 in 10,000 by 2006.

Prof Bishop said: "Our study shows pretty direct evidence to support the theory that changes in diagnosis may contribute towards the rise in autism.

"These were children that people were saying were not autistic in the 1980s, but when we talk to their parents now about what they were like as children, it's clear that they would be classified as autistic now.

"Criteria for diagnosing autism were much more stringent in the 1980s than nowadays and a child wouldn't be classed as autistic unless he or she was very severe.

"Now, children are being identified who have more subtle characteristics and who could in the past easily have been missed."

However, she warned against using the study to rule out a true rise.

"We can't say that genuine cases of autism are not on the increase as the numbers in our study are very small," she said.

"However, this is the only study to date where direct evidence has been found of people who would have had a different diagnosis today than they were given 15 or 20 years ago."