American researchers have revealed that 85 per cent of samples taken from autistic children with bowel disorders contain the virus. The strain is the same as the one used in the measles, mumps and rubella triple vaccine.

The findings will spark fresh concern about MMR, because they back theories of a causal link between the jab, autism and painful gut disorders suffered by a number of autistic children.

The study replicates findings made by the gastroenterologist Dr Andrew Wakefield in 1998 and Prof John O'Leary, a pathologist, in 2002.

Parents say their children were developing normally until they had the MMR jab, given when a child is between 12- and 18-months-old. The children now suffer from regressive autism.

One theory is that the virus passes through the gut, causing damage, and into the bloodstream, from where it is able to attack the brain.

More than 2,000 families claim that their children have suffered damage but the Department of Health reiterated last night that MMR is safe, a stance supported by the British Medical Association and all the Royal Colleges. Last year Government scientists failed to reproduce research results by Dr Wakefield.

Research to be presented this week in Montreal, Canada, provides fresh evidence that the measles virus is present in the guts of autistic children. Dr Stephen Walker, of the Wake Forest University School of Medicine, North Carolina, studied children with regressive autism and bowel disease. "Of the handful of results we have in so far, all are vaccine strain," he said.