Christopher Gillberg, professor of child and adolescent psychiatry at St George's Medical School, London, told the BBC there is little to suggest that MMR has anything to do with autism.

And the intense focus on the effect of the triple vaccine has stymied potentially far more fruitful avenues of research into a condition which is still little understood.

His comments come as the Department of Health launches a website to provide information to parents about MMR.

This whole MMR business has taken on proportions that have hampered research in autism

Professor Christopher Gillberg

A 1998 study showed one in 1,000 children were affected by autism. Experts say the figure is set to rise.

But Professor Gillberg, who will present a review of 40 major studies of the causes of autism to the annual conference of the National Autistic Society (NAS) in London on Friday, said that was because diagnosis and awareness had improved, not because there were more cases.

He said he had not seen "a shred of evidence" to link the measles, mumps and rubella vaccination, or any other immunisation, to autism.

Professor Gillberg said: "I think this whole MMR business has taken on proportions that have hampered research in autism.

"People are concentrating so much on disputing this or finding this or finding that in relation to MMR, when there has never been any strong evidence that this would be a road we should be travelling."

He added: "There are so many other clues to autism we have spent a lot of time in vain on debating this particular aspect because you can pick on anything and say we think this might be linked to autism and, interesting as it might be, I think you should follow the leads where there might be evidence."

He said autism was more likely to be caused by a combination of genetic and environmental factors.

Media hype

However, he says a link with MMR cannot be completely ruled out, and that the controversial study which first made the connection should be replicated before it can be ruled in or out.

Professor Gillberg said 10 years ago research had concentrated on whether facilitated communication was a cure for autism because that was being hyped by the media.

If we start looking at environmental issues that could cause autism, MMR should be considered

Jonathan Harris, Jabs

He said many expensive long-term studies had been carried out and shown it had no effect.

"But just because there was hype in the media everyone had to test it."

Jonathan Harris, a spokesman for Justice Awareness and Basic Support (Jabs), said research into autism must look at MMR.

"It must be acknowledged that there's a group of autistic children who do not conform to any of the present definitions of autism," he said.

"If anything good has come from the MMR issue, it's that the awareness of autism is greater.

"If we start looking at environmental issues that could cause autism, MMR should be considered as part of that."

The NAS says autism is being detected earlier, partly because babies are observed differently, such as through home videos, and partly because it is now known that early intervention can help.

Symptoms are often spotted when a child is between 18 months and two-years-old.

They can include difficulty communicating and an obsession with patterns and numbers.

Vernon Beauchamp of the NAS told the BBC: "We agree that if MMR could take too much priority and emphasis looking at this, that's not a good thing.

"There's a far wider range of areas which do need far more focus and research."