Fears about the safety of the MMR jab could dramatically increase the chances of outbreaks of measles, researchers have warned.

A study has shown that even a small drop in the number of children vaccinated can greatly raise the odds of an outbreak of the potentially deadly disease.

The fear-factor effect is greatest when vaccination levels are hovering just below the target needed to keep the disease at bay in the population as a whole.

Health check: A child is immunised against MMR

For example, the Swiss researchers calculated that if nine in ten children are vaccinated, there are unlikely to be any outbreaks of the disease.

But, if the nine in ten are vaccinated at a time when there are worries about the safety of the jab, the disease could thrive, with the number of cases similar to that expected if just seven in ten were vaccinated.

The researchers say vaccine scares can lead to clusters of unvaccinated children, making spread more likely than if the unprotected youngsters were spread evenly around the country.

Writing in the Journal of the Royal Society Interface, they said: 'Many high-income countries currently experience large outbreaks of vaccine-preventable diseases such as measles despite the availability of highly effective vaccines.

'We build on the growing evidence that belief systems, rather than access to vaccines, are the primary barrier to vaccination in high-income countries, and show how a simple opinion formation process can lead to clusters of unvaccinated individuals, leading to a dramatic increase in disease outbreak possibility.'

The warning comes as Britain mounts a vaccination campaign to head off a measles epidemic.

Around three million children and teenagers are believed to be at risk because they have either not had the controversial vaccination or not had the full course of injections.

Uptake of the MMR jab, which protects against measles, mumps and rubella (German measles), plunged after controversial research by Dr Andrew Wakefield linked it to autism and bowel problems.

However, dozens of other studies have since failed to find any link and the Government has repeatedly assured parents it is safe to vaccinate their children.

Professor David Salisbury, head of immunisation at the Department of Health, said: 'Parents who have not had their children vaccinated with the MMR vaccine should do so now.

'The evidence on MMR is absolutely clear – there is no link between the vaccine and autism.

'The MMR vaccine coverage is not high enough to remove the threat of recurrence of measles outbreaks. Measles is serious and in some cases it can be fatal. Delaying immunisation puts children at risk.'