England and Wales are facing the most serious outbreak of mumps since records began in 1995, official figures show.
Low uptake of MMR vaccine was blamed for the rise in mumps
Health Protection Agency figures show confirmed cases of mumps hit a high of 1,529 last year and look set to continue on an upward trend this year.
In the first quarter of 2004, there have already been 1,272 cases reported and 307 confirmed.
Most of the cases have been among young adults who missed out on the MMR programme that began in 1988.
More than 70% of the cases reported in the first three months of this year to the HPA were among this age group.
In Cornwall alone last month, at least 170 teenagers and young adults had contracted the illness.
Outbreaks have also been reported in Nottinghamshire and Derbyshire.
The HPA insisted that the rise in cases was not linked to the reduced take up of MMR amongst babies in recent years.
However, there are concerns that the drop in MMR uptake seen following claims the vaccine was unsafe will leave more people unprotected.
Some scientists have suggested the jab may be linked to autism and bowel disease.
However, no research has ever proved a link, and the overwhelming majority of experts believe the vaccine is safe.
A spokeswoman for the HPA said: "When MMR vaccination was introduced there were obviously children who were older than toddlers so were not routinely called up to be given both doses of MMR.
"Some of them may have been offered one dose and some would have been offered no vaccine at all.
"So you have a cohort of children who have now grown up and reached adulthood who are either poorly protected or completely unprotected against mumps.
"We have seen outbreaks over the last three to four years in that age group," she said.
She said the important thing to stress was that people can have the MMR vaccine at any age.
Mumps is an acute viral illness transmitted by contact with saliva from infected people
Symptoms begin with a headache and fever for a day or two before swelling of the parotid glands
Rarely, mumps has been linked with sterility in adolescent and adult males
Dr Natasha Crowcroft of the HPA's immunisation division said: "The HPA are recommending, along with the Department of Health, that school leavers and others in this age group, for example those starting university, who had not received MMR or had only had one dose, should be offered MMR."
The BMA said the rise was linked to poor uptake in general.
Take-up of MMR has declined since suggestions were made of a link between it and autism or bowel disorders.
The alleged association between MMR and autism or Crohn's disease has now been reviewed by the Committee on Safety of Medicines, the Joint Committee on Vaccination and Immunisation and the Medical Research Council.
All these found independently no evidence of a causal link between MMR and either Crohn's disease or autism.
A spokeswoman from the BMA said: "If you have got a group that misses out then I am afraid that outbreaks will sometimes happen.
"If uptake of the MMR vaccine continues to fall then further outbreaks will soon occur."