ONDON, Feb. 7 — The British government is struggling to quell fears about a common childhood vaccine that increasing numbers of parents believe may be linked to autism, despite strong official assurances to the contrary.

At issue is the so-called MMR vaccine, which immunizes children against measles, mumps and rubella, three potentially deadly diseases that were once common and have been mostly wiped out. The government says that the vaccine, which is administered in two doses, the first when babies are about 18 months old, is perfectly safe.

The issue has become a serious political headache for Prime Minister Tony Blair, who has been under pressure from the press as well as parents' groups to say whether his 20-month-old son, Leo, has had the vaccine. Mr. Blair has refused to answer but has strongly implied that Leo has been or will be inoculated, saying that he would be unlikely to tell other parents to do something that he was unwilling to do himself.

More and more parents, citing anecdotal evidence from parents of autistic children as well as several studies they say seem to suggest links between the MMR vaccine, autism and various bowel ailments, have decided not to allow their children to be inoculated. In London, only 75 percent of eligible toddlers are being immunized. The latest figures for the country as a whole show 84.2 percent are getting the vaccine.

In the United States, 90.5 percent of children ages 19 months to 35 months had been inoculated in 2000, the latest year for which figures were available.

A report issued in April by the Institute of Medicine, in Washington, a nonprofit research organization, said there was no evidence that the vaccine increased a healthy child's risk of developing autism.

The British government says that 95 percent of children need to be inoculated to give "herd immunity," and prevent the diseases from breaking out. Meanwhile, measles has been diagnosed in 11 children in a recent outbreak in south London. A further 18 are showing symptoms. The area of the outbreak — the boroughs of Lambeth, Southwark and Lewisham — has one of the lowest MMR immunization rates in the country, in part because of fears about the vaccine. Only 65 percent of eligible toddlers in these boroughs were immunized in the quarter leading up to September 2001.

The Conservative opposition said this week that the public had lost confidence in the safety of the vaccine. It called for the health service to offer parents the option of having their children inoculated against each of the diseases separately.

"In the circumstances, I think that reluctantly we have to accept that single jabs may be the only way of giving children protection," the Conservative health spokesman, Liam Fox, said. "At the moment, the choice for parents is MMR or nothing, and sadly, many of them are choosing — despite the evidence — to have nothing. I don't think that's acceptable."

Although the MMR vaccine was introduced in 1988, Britain offered separate measles injections until 1999. The government says the combined shot is safer and a more effective public health tool because, among other things, parents often fail to get the full number of shots for their children when the vaccines are administered separately.

In the House of Commons on Wednesday, Mr. Blair called concerns about the safety of the vaccine "scaremongering" and warned parents not to put their children at risk by giving in to "media hysteria." Some 90 other industrialized countries use the MMR vaccine, he said.

Mr. Blair's health minister, Yvette Cooper, told the BBC that the bulk of medical opinion holds that "the MMR is the safest way to vaccinate children against very deadly diseases."

She added: "In addition to that, they also tell us very clearly that introducing separate immunizations would reduce coverage, would lead to more children being unimmunized, would lead to more children being unprotected, more children at risk, more children falling sick, more children in hospital and more children dying and damaged as a result."

The possibility of links between autism and the vaccine has been the subject of many studies, and most researchers have concluded that there is no evidence of a connection. But a study to be published in April in the journal Molecular Pathology, based in Britain, found traces of the measles virus in gut samples from most children with autism and bowel disorders who were examined, but in only 7 percent of children without those conditions.

The British Department of Health said last week, however, that the study was flawed, in large part because it had no information about whether the subjects had been given the MMR vaccine.