Twice as many children may be affected by autism as had previously been thought, researchers claim today.

A survey conducted in 12 districts in south London suggests 1 per cent of children in Britain - about 100,000 - are affected by the lifelong developmental disability that impairs the way people communicate with each other. More than three times as many boys as girls are affected.

If confirmed, the finding will have big implications for health, education and social services. Previous estimates by the Medical Research Council suggested 60,000 children were affected. Prior to the 1990s, researchers estimated fewer than 5,000 cases.

Doctors from Guy's and St Thomas' hospitals, who conducted the study, said it was unclear whether the figure reflected a real increase in incidence, or increased awareness of the condition and better diagnosis.

Writing in The Lancet, Professor Gillian Baird, a specialist in autism and president of the Association of Speech Impaired Children, and colleagues said: "The consensus diagnosis of autism and in particular other autistic spectrum disorder cases could be associated with the broadening of diagnostic criteria over time, which might be responsible for the rise in reported prevalence, but other explanations cannot be ruled out."

Similar increases in autism and related disorders have been reported from around the world, but the reasons remain unclear. In a commentary on the latest finding, Hiroshi Kurita, of the Zenkoku Ryoiku Sodan Centre in Tokyo, said genetic factors were the most important cause of autism, but "no study has ever clarified the rising prevalence of pervasive developmental disorders from this aspect".

Two suspects among possible environmental causes are the measles mumps and rubella (MMR) vaccine, and thimerosal, a vaccine preservative containing mercury.

But Dr Jurita rejects these explanations. "Among many other compelling lines of evidence, the continuous increase in the incidence of pervasive developmental disorders after cessation of use of MMR vaccine in a northern district of Yokohama, Japan, and of thimerosal-containing vaccine use in Denmark, is a strong and overwhelming refutation," he said.

The south London survey of almost 57,000 children found the number of children aged 9 and 10 affected by autism and related disorders was 116 per 10,000. Previous estimates put the figure at 44.

The researchers identified more than 250 children already diagnosed with any form of autistic spectrum disorder, as well as 1,500 children on the special needs register whom, they hypothesised, might be unidentified cases.

They studied 255 children to whom they applied the latest diagnostic criteria. They concluded that 39 children per 10,000 had autism and 77 per 10,000 had a related disorder, making a total prevalence of 116 per 10,000, just over 1 per cent of the child population.

The National Autistic Society said the study highlighted the need for more research. It received over 35,000 calls from individuals with autism and their families last year, many unable to obtain help.

"This research underlines the importance of appropriate services for individuals with autism," it said. "Current provision is deeply inadequate given the scale of the need. Government and local authorities must ensure that education, health and social services are adequately funded and all staff appropriately trained. Autism is a lifelong disability and when an individual's needs are not met the long-term consequences both financially and for the individual's well-being are profound."

Sandra Dewar, 44: 'Diagnosis would have spared us harassment'

Sandra Dewar spent five years coping with criticism from teachers and parents over her son's behaviour - before he was finally diagnosed with Asperger's syndrome, a mild form of autism.

Stewart, now aged 13, struggled at nursery and mainstream school. He was denied the help he could have had if his problems had been recognised earlier.

"My friend was the first to suggest Stewart was autistic when he was aged two," says Ms Dewar. "Overnight he changed into this boy who didn't want to play, wouldn't settle, ran around and screamed. He didn't talk till he was three and a half."

When Stewart went to nursery, his behaviour did not improve. He couldn't mix with the other children and would attack them. The head of the nursery suggested he should have behavioural management. He started school at age five and was nearly expelled in the first week. His mother says: "He was suspended a couple of times, I was in and out of the headmistress's office and other parents were abusive to me."

She still did not know what was wrong and a year later she broke down at her GP's under the strain. Stewart was referred for specialist assessment - a battery of tests and interviews which lasted months. A year later, at age seven, he was diagnosed with Asperger's and two years after that his mother transferred him to a school for pupils with special needs.

Today, Stewart, who has a high IQ, is doing well academically and is expected to take his maths and science GCSEs a year early. But his social skills are those of a child half his age and he has no friends.

Ms Dewar says: "If he had been diagnosed before he started school at age five, we would have been spared all that harassment. There are programmes that can help children like Stewart but the earlier they start the better. When everyone is against you it is hellish. It is easy to see why some parents become suicidal."