Measles cases in England and Wales rose by 36% last year, according to the latest figures from the Health Protection Agency.
Confirmed cases rose from 990 in 2007 to 1,348.
Health Protection Agency immunisation expert Dr Mary Ramsay said the year-on-year increase was "very worrying". Most of the cases were in children not fully vaccinated with combined MMR and so could have been prevented.
The highest number of cases, more than 650, occurred in London, with only 15, the lowest number of cases, in the North East.
Dr Ramsay said: "There are still many children out there who were not vaccinated as toddlers over the past decade and remain unprotected.
"Unfortunately this means that measles, which is highly infectious, is spreading easily among these unvaccinated children."
She said the HPA was glad to see that public confidence in MMR has improved, with more than 8 out of 10 children receiving a dose of MMR by their second birthday. However, children who were not vaccinated many years ago are still at real risk.
"Measles should not be taken lightly as you can never tell who will go on to develop the more serious complications of pneumonia and encephalitis," added Dr Ramsay.
The HPA is urging parents to get their children vaccinated, stating that it is the "most effective way to protect children against measles, mumps and rubella".
The Department of Health established an MMR catch-up programme in August 2008, providing local primary care trusts and GPs with extra funding to identify children who had not been vaccinated.