The study, published in the US journal Pediatrics, says that women who have been inoculated while young have far less immunity to the disease to pass on to their own offspring.

If correct, this would mean that past decades of rising immunisation rates in the UK could have left unvaccinated modern babies increasingly vulnerable to the disease, which, in very rare cases, can kill.

The US Centres for Disease Control and Prevention said the finding reinforced the importance of early measles vaccination for all children with a high risk of exposure.

Vaccine for 36 years

Measles vaccine came into widespread use in the United States and elsewhere in 1963.

The report focuses on 128 unvaccinated infants who were exposed to measles from 1990 through 1993. The researchers had interviewed the mothers of the infants to find out if they had been vaccinated.

The government says there is no evidence that MMR is harmful

A third of the infants born after 1963 were infected, compared to 12 percent for infants of older mothers," said the study.

It concluded: "an increasing proportion of infants born in the United States may be susceptible to measles"

It said: "Most infants are protected from measles by passively acquired maternal antibody from birth until the antibody is depleted," the study said.

"The duration of the protection is dependent to a great extent on the amount of antibody received by the infant during pregnancy

"Women who have had measles disease have high measles antibody titers (concentrations), women who have not had measles but have been vaccinated effectively have lower antibody titers and women who have neither had measles nor been vaccinated effectively have no measles antibody."

Deliberate measles infection

In the UK, there has been a re-emergence of so-called 'disease parties', in which healthy children are deliberately exposed to children suffering from measles.

Afficionados claim that a healthy child should be able to fight off measles, and that the natural immunity acquired is beneficial.

However, the practice has been criticised by some GPs, who point out that measles is still dangerous and disabling in a small number of cases.

Although one million children die from measles world-wide each year, in the UK measles is rare.

However, complications are quite common. They include a severe cough and breathing difficulties (croup), ear infections, viral and bacterial lung infections (pneumonia), and eye infections (conjunctivitis).

Most are caused by secondary bacterial infections which can be treated with antibiotics.