Those affected now did not have the MMR vaccine as babies

Almost 1,400 cases were reported to the Health Protection Agency in the first 10 weeks of 2005, compared with around 1,700 in the whole of last year.

Experts say the cases are among young people who have missed the combined MMR jab, which was introduced in 1988.

They say those who have not been vaccinated should stay away from people they know with mumps.

Dr Meirion Evans, consultant epidemiologist with the National Public Health Service for Wales, said the rise in cases of mumps could be traced back to the beginning of 2004, when figures were quite high.

"Most of the people affected are older teenagers and young twenties and, thankfully, in that age group symptoms are by and large quite mild," said Dr Evans.

If you know someone who has got mumps and you know you have never had the jab yourself, stay away from them Dr Meirion Evans

"It's mainly explained by the fact that the MMR vaccine, which protects against mumps, came in at the end of 1988 so many people in this age group won't have had the vaccine offered to them as babies."

A small proportion of youngster today have not had the MMR jab and Dr Evans said that means there will be another group of teenagers in the future who have not been protected against mumps.

Dr Evans issued advice for people at risk of contracting mumps.

"Firstly, if you know someone who has got mumps and you know you have never had the jab yourself, stay away from them," he said.