Thousands of parents who refuse to let their children have the MMR vaccine are to be sent letters warning them that they risk causing the deaths of their family and other youngsters.

Health officials in Lanarkshire, prompted by a fall in the number of children having the measles, mumps and rubella inoculation, are writing to 1,800 families, warning parents of their individual and collective responsibilities.

Dr David Cromie, public health consultant for Lanarkshire Health Board and a leading light in the national campaign for MMR, said: "All it takes is for a child to pick up measles, say abroad on holiday, and within a week or two he or she will be spreading infection before symptoms occur, or a diagnosis can be made.

"Unprotected children who attend nurseries; childminders; playgroups; or Sunday schools are at particular risk, due to their contact with larger numbers of children.

"The idea of the letter is to inform parents that the drop in vaccinations means the risk of an epidemic is increasing, remind them that these are not harmless diseases but life-threatening illnesses, and explain that there is no scientific proof that the vaccine causes autism," he said. "The MMR programme is only effective if 95 per cent of the population are protected. There are some children in society who cannot have the vaccine because they already suffer from an illness which suppresses their immune system. Unless the people around them are protected they could die from measles," said Dr Cromie.

Concerns about the vaccination followed suggestions it may be linked to an increase in autism, bowel disease, asthma and other problems. Repeated scientific studies have failed to prove a connection.

More than 500 million doses of the MMR vaccine have been given in 90 countries over the past 30 years. Dr Cromie said: "The younger generation has forgotten how serious measles, mumps and rubella can be. A stark reminder came when two children died in the recent Dublin measles outbreak affecting 1,000 children."

Before immunisation was widely available, some half a million children in the UK caught measles each year.

* Downing Street moved yesterday to underline Tony Blair's commitment to the MMR jab, even though he refused to reveal whether his son Leo had been immunised.

Mr Blair refused to comment on Leo's medical record when it was raised during Prime Minister's Question Time in the Commons.

Despite claims in recent weeks that Mr Blair's wife, Cherie, was opposed to MMR, the Prime Minister's spokesman said she supported the policy. "The Blairs believe that their children are entitled to their privacy and that privacy is particularly germane when it comes to medical records."