Measles could soon become an ever-present threat to British children unless vaccination rates improve, say experts.
Measles can disable - or even kill
Researchers say that in the near future, the disease could become endemic in the UK, and the average outbreak of measles could involve a thousand children rather than dozens.
The latest study, by scientists at the Health Protection Agency and the University of London, is published in the journal Science.
One of its authors, Professor Vincent Jansen, said parents who did not protect their children against measles could be storing up major health problems for the future.
He said: "We expect this trend to increase over the next four years if vaccine uptake does not increase.
"We are now approaching the danger zone where measles could again become endemic in the UK."
The number of unvaccinated children in towns across the UK means measles - a potentially fatal or disabling disease - is close to becoming permanently established in the community.
That would mean an end to isolated outbreaks of the virus and the arrival of "self-sustaining" disease in which children could catch it at any time.
Researchers used a complex formula based on the number of unvaccinated children in a community, the average size of outbreaks and the age profile of the population to work out the risk of major outbreaks.
While there was also an element of chance involved, the results suggested that for the last four years, the UK was hovering close to the point at which such huge outbreaks became likely.
Experts said that the likelihood of larger measles outbreaks was already increasing but as many as 1,000 children might be affected by future outbreaks if vaccine uptake remained the same.
In an outbreak of that size, it is possible that a child might die from the disease - a recent 3,000 case outbreak in the Netherlands claimed three lives.
However, it is far more likely that significant numbers of children would be left with permanent disabilities by the illness.
A 1,600 case outbreak in the Irish Republic also killed three.
Professor Jansen said: "Parents should be aware that there are risks associated with the decision not to vaccinate and those risks are increasing.
"My hope is that this study will provide a warning signal."
While the vast majority of parents, even in the major cities, still chose to give their children MMR, more than one in seven in the UK did not receive the jab.
The unpopularity of the vaccine followed suggestions from some scientists that the combined jab may be linked to autism and bowel disease.
But no research has proved a link and the majority of experts believe the vaccine to be safe.
An unvaccinated child living in a town with 95% MMR coverage - the government target - enjoys "herd immunity" - protected to a great degree because vaccinated children around him or her cannot develop or pass on measles.
But in a town where coverage falls below 80% - such as some parts of London, parents cannot rely on this protection.
Private clinics offering single vaccines for measles, mumps and rubella said that it was "arrogant" for the government to suggest that MMR was the only solution to low vaccination uptake.
A spokesman said: "Single vaccines are widely available and extremely effective.
"If the Department of Health was really committed to offering the choices in healthcare that it so often talks about, this would extend to single vaccinations too."