Nursery schools will be told this week to warn parents when the number of children in their care with the MMR vaccination falls below recommended levels.

The National Day Nurseries Association (NNDA) will issue new guidelines to members asking them to alert parents to give them the option of taking their child out of the nursery.

The move comes after a member of the British Medical Association's GP committee called for children who have not had the triple injection to be barred from nursery schools. Dr Mustafa Kapasi, a GP in Greenock, Strathclyde, said parents should have to produce vaccination certificates before their children were enrolled.

"What we don't want is some kids getting measles or diphtheria and affecting the education of other children in those schools because their nursery may be closed, so I think it is not unfair to impose these targets on parents," Dr Kapasi said.

But Karen Walker, the strategic director of the NNDA, said excluding children from nurseries because they had not had MMR could be illegal under equal opportunities law. Many children could not have vaccinations because of allergies or family medical history and it would be unfair to deny them a nursery place, she said.

Instead, the association will advise nurseries later this week to warn parents when the number of children with vaccinations fell below 95 per cent, the level necessary to preserve "herd immunity" and prevent an outbreak. Ms Walker said: "We have had parents asking to know which children in the nursery have not been vaccinated. We will not say that, but we are telling nurseries that if levels fall below recommended levels they need to tell parents.

"That then gives parents the opportunity to decide if they want to take their child out of the nursery school."

Meanwhile, Sir Liam Donaldson, the chief medical officer, will start a publicity campaign this week to persuade every parent that MMR is safe and does not carry a risk of autism. He will send an open letter to clinics and hospitals challenging Dr Andrew Wakefield's suggestion of a link with autism.

"If I could sit down with every parent in this country and go over their concerns and explain it, I am convinced that I could reassure them not just of the overwhelming positive safety record of MMR vaccine but also the risks associated with a single vaccine which greatly increases the risk of children contracting these diseases," Sir Liam said.

But Roger Hayward, chairman of the Public Relations Standards Council, warned that the campaign "simply won't work" because parents would think the Government had failed to convince the media of its case. "There is a lot of evidence that advertising used in situations like this is actually counter-productive and worsens the situation rather than improving it," he said.