Parents' anxieties about the MMR vaccine were heightened yesterday after fresh evidence pointed to a link with autism and bowel disease.
Children suffering autism and a rare form of inflammatory bowel disorder have been found to have the same strain of measles in their intestines as the one in the controversial triple jab.
Although there is no proof that the measles, mumps and rubella jab has actually caused their autism or bowel disease, some experts say the discovery is 'significant'.
Campaign groups and researchers renewed their calls for single jabs to be available on the NHS.
Jackie Fletcher of JABS, which backs parents who believe their children have suffered due to MMR, said: 'This appears to be a key piece of the jigsaw, and there can no longer be any excuse for ministers and doctors to bury their heads in the sand.
'We need further research, but in the meantime the single jab should be made available to all who want it for their children.'
The research builds on earlier work by molecular pathologist Professor John O'Leary, of Trinity College, Dublin, which identified the presence of measles in the guts of children with developmental disorder without identifying the particular strain of the disease.
But the latest research led by Professor O'Leary pins the measles lodged in their guts down to MMR vaccination. Fragments of the MMR measles virus were found in 12 autistic children.
The new findings are due to be presented at the Pathological Society of Great Britain and Ireland next month.
Dr Andrew Wakefield, the specialist who first raised concerns about MMR in 1998, said yesterday: 'The Government says there is no evidence of bowel disease in these children and they are normal.
'Now it has been found that they have the vaccine strain of measles in their gut. That is significant. The Government are running out of places to go on this.
'There can be no justification for not making the single measles jab available.'
Dr Wakefield, who was effectively forced out of his job at the Royal Free Hospital in London, revealed he is to carry out studies on more than 30,000 children who have been given the single measles vaccine-Working with the Direct Health clinic in London, he aims to examine rates of autism and bowel disorder in the large group of children who have never had MMR and compare them with rates among those who have had the triple jab.
The work will be carried out in America, where Dr Wakefield is now based.
Last night, the Department of Health insisted MMR was safe - a view backed by the
British Medical Association, the Royal College of Nursing, the Royal Colleges of Paediatricians and GPs, and the Community Practitioners and Health Visitors' Association.
A spokesman added: 'There is no proven link between MMR, autism and bowel disease and as for its safety, we would point to a BMJ research article last week which looked at more than 2,000 studies and found no evidence of any link.'