MERCURY in vaccines for babies and infants could be the cause of a steep rise in cases of autism in children around the world, according to a growing number of scientists.

The increase in reports of autism in Britain, America and some other countries coincides with a growth in the number of inoculations given to young children, say the researchers.

Many of the vaccines contain a preservative called thiomersal, which is 49.6% mercury - a substance known to have neurotoxic effects, especially in infants whose brains are still developing. Symptoms are similar to those displayed by autistic children.

Autism is a spectrum of disorders which range in severity from bizarre, violent behaviour to an inability to communicate or interact socially, along with repetitive patterns of behaviour.

Estimates of the prevalence of the syndrome in Britain range from 10 cases per 10,000 of the population with "classic" autism, to 9.1 cases per 1,000 showing some signs of autistic behaviour.

The National Autistic Society estimates that there are about 500,000 people with autism in Britain, 120,000 of them children. According to one recent study, there has been a tenfold increase among children between 1984 and 1994.

The new possible explanation of the rise has emerged after a two-month review of all the available information by The Sunday Times. Several groups of academics and researchers in America and Sweden are investigating similar theories that a combination of factors is to blame.

They include a genetic predisposition, the cumulative effects of mercury in vaccines lowering the immune system, with the controversial measles, mumps and rubella (MMR) triple vaccine being a possible trigger.

The US Institute of Medicine is so concerned that officials have organised a two-day meeting in July to discuss the "hypothesis" that thiomersal and mercury in vaccines are linked to autism.

Boyd Haley, chemistry professor at the University of Kentucky, has been asked to submit a paper. "Thiomersal is extremely toxic. The preliminary data is convincing and does indicate that vaccines are the most likely suspect for causing autism," he said.

In general, the researchers argue, the cumulative effects of mercury impair brain development and damage the child's immune system and gastrointestinal tract, resulting in hypersensitivity to toxic environmental substances.

This build-up could lead to autism or a form of mercury poisoning - whose symptoms are similar. In addition, researchers believe, the MMR triple vaccine, usually given at 18 months to two years, could trigger autism because the damaged immune system cannot cope with three live viruses at once.

Only some children exposed to mercury will develop symptoms. Researchers believe this indicates that there may be a genetic predisposition. This theory was reinforced by a study published this month which showed that in 99% of autistic children a family of proteins essential for disposing of mercury and other heavy metals is missing or "disabled".

The proteins, called metallothionein (MT), are the main way in which the body counters heavy metal. The study, by the Chicago-based Pfeiffer Centre, a health research institute, found that of 503 autistic patients 499 showed an MT "dysfunction".

The number of vaccinations given to babies and children in Britain and America has increased significantly. In the United States the number given before the age of two has risen from eight in 1980 to 22 now.

In Britain in 1970, most children received diphtheria, tetanus, polio, whooping cough and BCG for tuberculosis; about half were also immunised against measles. In 1972 rubella was added; MMR in 1988, Hib (Haemophilus Influenza type b), against a form of meningitis in 1992, MMR as a second dose in 1996, and meningitis C in 1999.

The MMR first dose is given between 12 and 15 months, with diphtheria and tetanus and the second dose of MMR at three to five years.

Like tens of thousands of other children, Melissa Wickens, 10, underwent a full course of inoculations. When she was just 14 months, after she had received her MMR jab, her life changed.

From being an alert, normal child, her behaviour became erratic. Typically, when she arrives home from school she throws down her fluffy red school bag and pulls everything out of it. In the kitchen she snatches a box of breakfast cereal and crams flakes into her mouth. Later she smears strawberries over herself and her bedroom upstairs.

She has already broken several television sets and throws chairs around the sitting room and pulls down shelves. Melissa, who was given the MMR vaccine in 1992, has severely regressive autism and inflammatory bowel disease.

"On the same day as the MMR she started a high-pitched screaming," said her mother Marion Wickens, from Brighton. "Over the next few weeks she cried inconsolably, started to bite herself and to pull out her hair, resulting in bald patches on her head.

"She used to have a lovely sparkle in her eyes but they went blank. She couldn't understand 'Where's mummy?' any more. It no longer registered."

She concluded: "Melissa wasn't born autistic, she regressed."

MMR does not contain thiomersal, though other child vaccines do. Thiomersal was introduced in the 1930s as a preservative and went into common use without review by America's Food and Drug Administration (FDA) because it was assumed to be safe.

In America, researchers found some infants who are being vaccinated using multidose vials with thiomersal can receive 62.5 micrograms of mercury per visit. This is 100 times more than the intake considered safe for the average six-month-old by the US Environmental Protection Agency.

In June 1999 the FDA discovered that: "Infants who receive thiomersal-containing vaccine at several visits may have been exposed to more mercury than recommended by Federal guidelines." The following month the European Agency for the Evaluation of Medicinal Products (EMEA) issued a statement saying: "Cumulative exposure to ethylmercury [found in thiomersal] . . . could lead to a potential cause for concern."

In May last year, a scientist from the US Centres for Disease Control and Prevention (CDC) gave a presentation, based on a study of the Vaccine Safety Datalink Project - a database of 400,000 children - with evidence of harm. Dr Thomas Verstraeten of the CDC found the screening analysis suggests "statistically significant associations" between certain neurologic developmental disorders - such as attention deficit disorder, and speech and language disorders - and exposure to mercury from thiomersal-containing vaccines before the age of six months.

As a result, the EMEA issued another statement last June, saying: "For vaccination in infants and toddlers, as a precautionary measure it would be prudent to promote the general use of vaccines without thiomersal. Moreover, the use of thiomersal-free vaccines should be recommended for newborns."

Neither of these EMEA statements received national newspaper publicity in Britain, and at least 10 thiomersal-containing vaccines for children are still in use as drug manufacturers are permitted to finish stocks.

Jane Maroney El-Dahr, an immunologist at Tulane University medical centre in New Orleans, said: "It is important for me to emphasise that the message is not to not vaccinate children, but to make sure that vaccines are thiomersal-free."

The health department said: "Thiomersal has played an important role as a preservative in vaccines.

Because thiomersal contains mercury, both European and American regulators have recently recommended that vaccine manufacturers phase out its use wherever possible as a precaution. They have not recommended the withdrawal of any vaccines.