Parents are urged to ensure their children get the MMR jab
Measles cases in England and Wales rose by 36% in 2008, figures show.
Confirmed cases increased from 990 in 2007 to 1,348 last year - the highest figure since the monitoring scheme was introduced in 1995.
Health Protection Agency experts said most of the cases had been in children not fully vaccinated with combined MMR and so could have been prevented.
Immunisation expert Dr Mary Ramsay said the rise was "very worrying", adding measles "should not be taken lightly".
More than 600 of the 2008 measles cases occurred in London, where uptake of the vaccine for MMR - measles, mumps and rubella - is particularly low.
Public confidence in the triple MMR vaccine dipped following research - since discredited - which raised the possibility that the jab may be linked to an increased risk of autism.
It led to some parents opting to pay privately for single vaccines.
Across the UK, 84.5% of two year olds have been immunised with their first dose of MMR.
But by age five, when children are recommended to have a second dose, the latest uptake figures are 77.9%.
Since 2005, the number of cases of measles has been rising year on year.
There have also been sporadic outbreaks of mumps in recent years.
Last summer, England's Chief Medical Officer, Sir Liam Donaldson, announced a catch-up programme for those who had not received one or more doses of MMR.
The Department of Health, which said the latest figures were concerning, has provided extra funding to PCTs and additional supplies of vaccine.
Dr Ramsay said: "There are still many children out there who were not vaccinated as toddlers over the past decade and remain unprotected.
MEASLES CASES BY AGE
Under 1 - 112
1 to 3.5 years - 265
3.5 to 11 years - 432
12 to 18 years - 286
18 years and over - 252
Age unknown - 1
"Unfortunately this means that measles, which is highly infectious, is spreading easily among these unvaccinated children.
"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."
She said it was "never too late" to get the MMR vaccine.
The figures come as a report from the World Health Organization on a measles outbreak in Germany in 2006, in which two children died, found 80% of those who had caught the infection were unvaccinated.
Dr Peter Strebel, an immunisation expert at the WHO, stressed that even in countries with good health services, measles could be very serious.
"Parents and doctors need to be reminded that measles is a highly contagious disease," he said.
"Even healthy and well-nourished children, if unvaccinated, are at risk of measles and its complications such as pneumonia, encephalitis and, although rare, death."
Professor David Salisbury, director of immunisation at the Department of Health, said it was "irresponsible" for parents not to have their children vaccinated.
He said: "I think it's irrational, I think it's putting children's lives at risk. I can see no shred of benefit.
"There is no evidence that having vaccines separately is better. There are good reasons why it's worse."
Professor Steve Field, chairman of the Royal College of GPs, said confidence in the MMR vaccine was returning but it was vital that parents made sure all their children had received both doses.
"Measles is a sinister and nasty illness and shouldn't be taken lightly."
A spokeswoman for the Royal College of Paediatrics and Child Health said: "We cannot stress too strongly that all children and young people should have the MMR vaccine.
"Overwhelming scientific evidence shows that the vaccine is safe."
Figures for the number of measles cases in Scotland for 2008 are expected to be released in March.
There had been a small increase earlier in the year due to a cluster of 54 cases in Greater Glasgow and Ayrshire.
Take-up of the MMR vaccine in Scotland though is running at around 95% for the first dose.
Northern Ireland has reported 24 probably cases of measles in 2008.