Dublin-based researcher Dr John O'Leary, working in collaboration with Dr Andrew Wakefield, reported at the weekend that they had found traces of the strain of the measles virus used in MMR in the guts of a small number of autistic children.

However, a government scientist has hit back, saying that requests to Dr Wakefield for information that would support his research have been ignored.

His suggestion was that unless the researchers were prepared to throw their techniques open to scrutiny, their findings would carry less weight.

He said: "We can speculate forever about what it might mean - but what we need is a better understanding what has been done and how it has been done."

Sample checks

He said that Dr Wakefield needed to supply evidence that the samples of gut tissue had not become cross-contaminated.

"Four months on we still don't have an answer to the questions that we posed - and these questions are crucial."

The latest research has not yet been formally reviewed and published in a medical journal.

Autism Research Campaign for Health (Arch) is demanding an end to government publicity claiming MMR is "indisputably safe".

Research call

Jackie Fletcher, from the pressure group Jabs, described the latest research as "another piece in the jigsaw".

She called for more research into the safety of MMR.

Dr Salisbury said that the government and Medical Research Council was already funding new research into the vaccine.

A study published last week in the British Medical Journal, described as the "most in-depth analysis to date", found no evidence the triple vaccine was linked to the illnesses.