A measles outbreak in south London has raised fears about the number of parents shunning the controversial measles, mumps and rubella vaccine.

Three children - two under a year old - have been confirmed as having the illness and 22 others have undergone tests for which the results will be known next week.

The two younger children with measles are under the age at which the vaccine is first given but it emerged the others, many of whom attend two nursery schools in Streatham, have either not had the jab at all or not yet completed the course of vaccinations.

The local health authority has advised parents to get their children inoculated as well as giving them information about measles symptoms. Uptake of the MMR vaccine in the Lambeth, Southwark and Lewisham area, which includes Streatham, is among the lowest in the country, with a figure of 72.3% during one quarter last year.

The most recent figure for London as a whole is 73.4% compared with 84.2% nationally, well below the 95% target which would give "herd immunity". The health department says the overall figure is expected to improve slightly soon.

Last month health officials in Halton, Cheshire, said the uptake there had fallen to a "dangerously low" level of 77% but the government does not want to be accused of scare tactics by opponents of the vaccine. This could both confuse parents and make them feel guilty.

Nevertheless ministers have been desperate to fight back against those who warn that the vaccine might be linked to autism and bowel disease. Tony Blair resisted calls to say whether 20-month-old Leo had had the jab, which is given to children between 12 and 15 months with a booster between three and five.

However, he said last month: "I believe it is safe. I would not be saying to people we advise you to do this if I thought it was dangerous for my own child."

The Health Department said last night that it was not just children who had not had the jab who were at risk of catching the disease but those below the age at which it was given. "It is important health authorities encourage people to take up MMR, which is the safest way of protecting against these diseases."

A spokesman said take-up in London had been historically low because of its mobile population and problems of some health authorities failing to call people in for their jabs. "The national rate is stable," he said. "It may not be as high as we want but it is not collapsing."

Last year there were 2,466 suspected cases of measles in England and Wales and 74 cases confirmed.