Blair's baby was given MMR vaccine last week

Tony Blair's son Leo was given the MMR vaccine last week, The Independent learnt yesterday, as health officials disclosed that the number of babies being immunised was lower than ever.

The Prime Minister has refused to bow to pressure to say whether 20-month-old Leo has had the injection. He and his wife, Cherie, fear that if he does the family will be drawn into revealing other details of their four children, whose privacy they want to protect.

Concerns over a low take-up of the vaccine were heightened yesterday by an outbreak of measles at a private nursery school in Clapham, south-west London. Three cases had been confirmed with 22 suspected cases being tested, the Department of Health said.

None of the infected children has been given the MMR jab, which protects against measles, mumps and rubella but which has been controversially linked to autism.

Newspaper reports shortly before Christmas suggesting that Leo had been vaccinated were wrong. The belief is that the Blairs always intended Leo to have the vaccine, but delayed because he was unwell. The Blairs were reluctant to explain this publicly because they feared it would amount to giving a "running commentary" on their children's health. Downing Street, which has consistently refused to disclose whether Leo has had the vaccine, declined to comment last night. The Prime Minister's official spokesman said: "He has made it clear that he fully supports the policy [of encouraging parents to have their children vaccinated]. In his view, the scientific evidence is clear. He has been completely unequivocal about that."

Downing Street aides noticed a subtle change in Mr Blair's approach on the Jimmy Young programme on BBC Radio 2 on 24 January. Leo is thought to have had the jab two days earlier, when the Blairs were in London.

The Prime Minister refused to say whether his son had had the jab. But he said: "We certainly would not ask anybody or say or advise people to have this vaccine if we thought it was the wrong thing for our child. Now I hope that people understand what I'm saying there, but I'm not going to get into the situation of answering a whole lot of details about what treatments Leo has."

Yesterday the Public Health Laboratory Service disclosed that the take-up of the MMR jab was 84.2 per cent, the lowest figure since monitoring began in 1995. The target set by the World Health Organisation for successful immunisation is 95 per cent. In London, the figure is only 73.4 per cent. The highest take-up is 89.3 per cent in Northern Ireland.

The Lambeth, Southwark and Lewisham Health Authority said it was investigating 25 cases of viral illness in the area. Three had been confirmed as measles. "In all of these cases, the children have either not had their MMR vaccine or have only had one dose," it said.

A Department of Health spokesman said: "The national [take-up] rate is stable. It may not be as high as we want but it is not collapsing."

Liam Fox, the shadow Health Secretary, said: "The Government's immunisation policy is a public health disaster. Labour health ministers have simply failed to grasp the importance of establishing public confidence in the MMR vaccine."