he MMR jab 'significantly' increases the risk of developing autism and other brain and nerve disorders, according to a study.

U.S. experts say the vaccine may be a factor in up to 15 per cent of such conditions.

The findings were hailed last night by campaigners who say the triple MMR vaccine is behind the rising number of youngsters with bowel disorders and autism.

They say it is the first time there has been an authoritative analysis of data on adverse reactions to vaccines and claim it vindicates both their warnings and the alert sparked by British expert Dr Andrew Wakefield five years ago.

The news will increase pressure on the Government to make single vaccines against measles, mumps and rubella freely available.

Jackie Fletcher, who set up the support group Jabs, also called for a full research programme into MMR.

Use of the controversial vaccine in Britain is already at a record low of 80 per cent among two-year-olds, when at least 95 per cent is needed to prevent measles outbreaks. But the Department of Health continued to insist last night that studies worldwide had shown the vaccine was safe.

The new research, by a team led by top geneticist Dr Mark Geier, involved analysing information on an official U.S. database showing vaccine reactions between 1994 and 2000.

Dr Geier found a 'statistically significant' increased risk of children suffering a range of disorders within five to 20 days of an MMR jab.

These included autism, mental retardation and encephalitis, which causes brain inflammation.

Dr Geier, who has published over 70 scientific articles and is regarded as an expert on vaccines in the U.S., Canadian and English courts, said: 'The studies tend to confirm and extend the work by Dr Andy Wakefield that caused so much concern in the UK.'

Details of the study, to be published in the medical journal International Paediatrics, were released yesterday in the doctors' newspaper Pulse.

Dr Geier has also carried out research into the effects of a mercury-based preservative called thiomersal which is used in childhood vaccines.

He is due this summer to publish further research which claims to show a causal link between mercury in vaccines and brain disorders.

Dr Geier has also written to U.S. senator Hillary Clinton, who is heading a review of American vaccine compensation policy, warning that a 'tragic and massive autism epidemic' is under way and MMR may be one cause.

His letter says: 'Our best estimates are that thiomersal contributed to about 75 per cent of the cases of neurodestudiesvelopmental disorders while the MMR contributed to about 15 per cent.'

Mrs Fletcher, who believes her son Robert, now nine, was seriously damaged by MMR, said monitoring for side effects was less thorough in the UK than in the U.S., where children cannot enter school without proof of vaccination.

But Dr Elizabeth Miller, head of the immunisation division at the Government's Health Protection Agency, said: 'The research of this group is well known, and they have published a number of using the Vaccine Adverse Events Reporting System database.

'This database consists of voluntary reports, similar to the yellow card scheme in this country. Such reports are not designed to test hypotheses and great care needs to be taken in analysing such data to avoid generating flawed results.'

A Health Department spokesman said the Committee on Safety of Medicines had advised that this type of analysis cannot be used to determine and compare the incidence of adverse reactions associated with different vaccinations.

He added: 'Therefore, because of serious methodological flaws, the conclusions of the authors concerning the association between MMR and the outcomes studied cannot be justified.

'The CSM has fully investigated the claims of a link between MMR vaccine and autism. The view of the CSM remains that the available scientific evidence does not support a causal link between MMR vaccination and autism. This view is in line with the consensus of several national and international scientific organisations.'