Challenged directly for the first time on whether 18-month-old Leo has been given the controversial triple vaccine, the Prime Minister stonewalled.
'I'm afraid I'm not going to enter into any public discussion on the health of my children,' he told the Commons.
Official Government policy is that the combined measles, mumps and rubella vaccine is quite safe but some critics claim it may be linked to autism and bowel disease.
Parents have been left with an agonising decision over whether to allow their babies the jab. The fears have led to intense pressure on Mr Blair to say whether he and his wife have actually followed Government policy with Leo.
But Mr Blair adamantly refuses to do so, citing his desire to protect the privacy of his family. His silence has inevitably fuelled speculation that he and Cherie may have opted for Leo to have a single measles jab, which has to be done privately and can cost up to £200.
The controversy is causing growing concern at the Department of Health, where officials fear it may be undermining the whole MMR policy and putting more children at risk from infection.
The issue was raised at Prime Ministers' Question Time yesterday by Tory Julie Kirkbride, the mother of a 14-month-old boy, who insisted the public had a right to know what the Blairs were doing.
She pointed out that the Government was conducting a vigorous campaign to inoculate all children with the triple vaccine and was determined to prevent parents from giving their children single vaccinations.
Miss Kirkbride, who has decided not to give her son the triple jab, went on: 'Does he not see his legitimate desire to protect the privacy of his child is very much at odds with legitimate public interest on this matter who want to know whether you practice what you preach.
'Will he take this opportunity to let us know whether little Leo has had his MMR jabs and in doing so reassure parents?'
But the Prime Minister refused. Instead, he listed the organisations that support the MMR vaccine - the World Health Organisation, the British Medical Association, the Royal College of General Practitioners, the Royal College of Paediatrics and Child Health, the Royal College of Nursing and the Community Practioners and Health Visitors Association.
He added: 'We fully support the campaign that is being mounted at the present time.'
The triple jab is given on the NHS to babies between 12 and 18 months old, with a booster a couple of years later. Introduced in 1988, it has virtually eradicated measles, mumps and rubella in immunised Western populations.
But concerns about possible sideeffects have led to increasing numbers of parents deciding against the jab.
The latest figures show the proportion of 15-month-olds being given the vaccination is now 84.2 per cent, the lowest since it was introduced. The fall has raised fears that the infections may begin spreading again.
Mrs Blair is known to be interested in the issue of possible side effects. The Daily Mail has revealed she has responded to e-mails from two concerned mothers. She has also attended charitable events for the National Autistic Society.
The Prime Minister's spokesman insisted last night, however, that the public has no right to know anything about the medical records of any of the Blair children.
'The Prime Minister and Mrs Blair have been pretty fundamentalist about protecting their children's privacy from the word go for reasons that are well known to you,' he told reporters.
'This is not something that should come as a particular surprise, particularly when it comes an individual's - albeit a very small individual's - medical records.'
The spokesman went on: ' Everyone is entitled to their privacy. That is particularly germane when it comes to medical records.
'There is a stated policy on MMR that has widespread support from august medical establishments and the Prime Minister and Mrs Blair support that policy. But that does not make it an open season for them to have to answer questions about Leo.'
Told that the refusal to answer would create the suspicion the Blairs have not given Leo the MMR jab, the spokesman replied: 'I am not commenting either way. But you can just imagine what people would say if we made a habit of answering these questions. It is the thin end of the wedge.'
He said the Blairs were pursuing a 'point of principle', adding: 'They support the Government's policy on MMR. They should not have to answer detailed questions.'
Mrs Blair's half-sister Lauren Booth, who told recently how she refused to let her 12-month-old daughter Alexandra have the MMR jab, cast doubt yesterday on whether parents can trust the Government over vaccines.
Miss Booth, who has joined calls for the Blairs to start an 'honest debate' about MMR safety, said: 'I think it is a very strange time to be a parent now.
'The Government wants you to check the labels on bottles, they want you to make sure there is less salt in the diet. But at eight weeks and even younger, you are supposed to inject them with industrial-strength chemicals and heaven knows what. Every week, it seems, there is a new study out proving a link.'
Speaking on the Richard & Judy programme on Channel 4, she added: 'We don't know what's in these vaccines. The fact they draw from calves' foetuses... it is awful things we are putting in children's bodies.'
'I think the focus on Cherie and Leo keeps happening because we don't feel we are getting enough information in other ways from the Government.'
We will never get the truth ,but if a person has something to hide then they will never tell the truth they will skirt around the truth if there is something to hide and we all know Mr T. Blair as the most untrustworthy person on the planet, if he refuses to discuss this very important thing then the obvious result is no the baby didn't have the mmr and it was hidden from the general public .So obvious is this fact its a case of don't do as i do i am rich enough to bypass normal laws they don't apply to me .So he is saying this injection is dangerous to children
