Sussex is on the brink of a measles epidemic because too few children are being vaccinated.
In Brighton and Hove, one in four youngsters are not being given the measles, mumps and rubella (MMR) vaccine.
The take-up rate of 73 per cent is one of the lowest in England, new research has revealed.
In East Sussex one in five babies are not being vaccinated because of parents' fears about a possible link between the triple-vaccine and autism.
In West Sussex the take-up rate is 88 per cent, in line with the national average.
The Department of Health, which insists the controversial jab is safe, has warned of an increase in the number of cases of measles in the past 12 months.
However, worried parents are pushing for the introduction of single vaccines for each of the three potentially-fatal childhood illnesses to be made available on the NHS.
In Brighton and Hove, a private clinic offering the individual jabs was deluged with applications and Healthchoice UK is now planning a two-day session at a larger venue.
Vincent Jansen, who wrote the new report for the Wellcome Trust, said: "My hope is that this is a warning for the parents. We are approaching the danger zone where measles could once again become an endemic disease in the UK."
The Department of Health said MMR was still the best form of protection and recognised by the World Health Organisation as having an outstanding safety record.
A spokesman said: "There is no credible scientific evidence showing an association between MMR and autism.
"We don't believe giving single vaccines is the right thing to do. Giving children separate vaccines spread out over a period of time would be dangerous, leaving children vulnerable to catching those diseases while they remain unimmunised.
"No country in the world recommends MMR to be given as three single vaccines."
Jackie Fletcher, national co-ordinator of Jabs, a support group for children damaged by vaccines, said the new research was very worrying.
She said: "We have got to a situation where the health minister must step in and reinstate the single vaccines on the NHS so parents have the choice.
"The same drugs company that manufactures the MMR also makes single vaccines, so all it needs is for the health minister to order the vaccines into this country and make them available on the NHS."
Jenny Kinsella, of Ditchling Road, Brighton, paid £240 to have her son Tommy, 18 months, protected with single vaccinations.
She said: "I thought long and hard before making this decision and I simply felt I was not happy with the MMR jab."
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