People aged 11-25 may not have had an MMR vaccination

It is aimed at those aged 11 to 25, who did not catch mumps when they were younger, or received the vaccine, which was introduced in 1988.

There have been 3,000 reported cases of mumps so far this year. In 2001 there were just 72 cases.

Free MMR jabs will be offered at universities and GP surgeries.

Public health experts have warned that many young people are at risk of contracting mumps because they have not been given the combined measles, mumps and rubella vaccination.

MUMPS FACTS Mumps is an acute viral illness transmitted by contact with saliva from infected people Symptoms begin with a headache and fever In some cases mumps has been linked with sterility in adolescent and adult males

The campaign will offer two doses of MMR vaccine to those still in schools and up to 18 years of age, and one dose of MMR to those between 18 and 25 who are not sure whether they have received the jab before.

The MMR immunisation is estimated to protect about 95% of people who receive one dose, and around 99% of those who receive two doses.

Dr Richard Roberts, head of the immunisation programme at the National Public Health Service for Wales, said it was a very safe vaccine and was a good opportunity for many vulnerable young people to get protection from measles, mumps and rubella.

"Those in education and aged 11 to 25 should wait to be contacted through schools and colleges," he said.

"Those aged 16-25 not in education should ask at their local surgery about a free MMR jab.