Eighteen cases of measles have been confirmed and a further 19 are suspected in an area of South Yorkshire, it has emerged.
All those affected are children who have not had the MMR innoculation. Some are too young to be immunised.
Health officials in Doncaster warned there could be a large outbreak in the town when five cases were confirmed last week.
There has not been a confirmed case of measles in the area for five years.
Autism fears
Parents are being urged to let their children have the measles, mumps and rubella vaccine.
The immunisation was introduced in the UK in 1988 and by 1992 more than 90% of children were being immunised.
Children are given a first dose at 12 to 15 months and a second, booster dose, at between three and five years old.
But about 2,000 families have taken legal action claiming their children have been damaged by the MMR jab, with many believing it has triggered autism.
In December last year, Julie Kirkbride, Conservative MP for Bromsgrove launched a bid to allow children to be immunised separately against the three diseases
But the Department of Health said there is no evidence to suggest that separating the vaccines is safer.
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