And they emphasised that there is no link between bowel disease and the jab.

The BBC's Fergus Walsh: "Two major studies have ruled out any link with autism or bowel disease"

Experts have also warned if confidence in the vaccine, known as MMR, does not return, new epidemics of childhood diseases could break out in as little as three years.

Two new studies published in "The Lancet" on Friday show there is no link between the introduction of the vaccine and the sharp rise in autism diagnosed in children.

Doctors and government health experts are now hoping the new findings will convince parents who have shunned the jabs that they are safe.

'Mothers must trust MMR'

Professor Brent Taylor, of the Royal Free Medical School in north London, which carried out the research, said it was important mothers began to trust MMR again.

BBC News' Richard Hannaford talks to the mother of one autistic child

He said: "There are a large number of children who are presently at risk from these damaging and occasionally fatal diseases.

"I do hope our results will be able to reassure parents and other concerned people about the possibility that the MMR may cause autism."

Children can still be vaccinated up to three years old

A connection between MMR and bowel diseases such as Crohn's Disease has already been ruled out by government experts.

Earlier studies looking for any traces of the measles virus in the guts of Crohn's sufferers yielded nothing, suggesting the virus was definitely not responsible for the disease.

No difference in autism rates

Dr Jeremy Metters: "If MMR is not taken up, there could be an epidemic of disease"

The health scare surrounding MMR and autism has been raging since the publication in February 1998, also in "The Lancet", of small-scale research from the Royal Free Medical School speculating on a link with autism, a condition which damages behavioural development in children.

But the new research, conducted into almost 500 children with autism in the North Thames health region, showed no difference in the prevalence of autism between those who had been immunised and those who had not.

In fact, the rise in the number of children diagnosed with autism pre-dated the introduction of the MMR vaccination in 1988.

Neither was there any statistical link between the age at which autism developed and the time of vaccination, suggesting that giving the jab did not act to hasten the development of the condition.

Worrying drop in vaccination

But new figures show the proportion of children receiving the vaccine at 16 months has fallen from 85% to 75%.

Dr Elizabeth Miller says there is no link between MMR and autism

Dr Elizabeth Miller, of the Public Health Laboratory Service, said the large drop in the number of children under 16 months being immunised meant there was now a genuine risk of the first measles epidemic for a decade.

The last one was responsible for the deaths of 17 children in the UK.

She said: "If the current level of coverage continues, in three or four years time there is a risk of a serious resurgence."

Plea for missed children

She urged parents who had missed the nominal 16-month time limit to take their children to the GP anyway.

She said: "The child can be vaccinated at any time before the age of two or three."

An explanatory leaflet has been produced and will be distributed to concerned parents by GPs.

Deputy Chief Medical Officer Professor Jeremy Metters said it was impossible to completely disprove a link between MMR and autism, and conceded that medical research had no explanation for the meteoric rise in autism cases over the course of the late 80s and 90s.

Professor Arie Zuckerman, the Dean of the Royal Free Medical School, conceded that the findings of the original report had been overstated to the press, sparking the original scare.

He said: "The fact that some of the authors went beyond the original paper was a matter of concern to the Royal Free."

But some mothers are still convinced of the link between autism and MMR.

One, interviewed by the BBC, said of her own child: "He was a normal happy boy until 13 months old, when he had the MMR injection.