Parents in England and Wales are being urged to have their children vaccinated after a tenfold rise in measles cases in the first four months of the year.
The Health Protection Agency reported 334 cases compared with 33 in the similar period last year.
The outbreak is thought to be linked to an epidemic in France, where 7,000 cases have been reported since January - more than in the whole of 2010.
The HPA says it is "crucial" that those at risk are fully immunised.
It says the latest cases are mainly among unvaccinated people under 25 years old and are centred on "small clusters in universities, schools or families or associated with travel abroad".
Worst-hit are London and the South East, with 104 and 102 confirmed cases respectively in the first quarter of this year.
Recently the Health Protection Agency sent out letters to some primary schools and further education colleges in London warning of the risks of taking children who are not fully immunised to mainland Europe.
Dr Mary Ramsay, head of immunisation at the Health Protection Agency, warned parents and young adults of the importance of immunisation.
"Although MMR coverage has improved over the last few years, we cannot stress enough that measles is serious and in some cases it can be fatal.
"Measles is a highly infectious and potentially dangerous illness which spreads very easily.
"Whether you stay here in the UK or travel abroad, it is crucial that individuals who may be at risk are fully immunised."
In the UK, two doses of the MMR vaccine are usually given, the first at around 12 months and the second around the time of starting school.
The HPA advises parents to contact their GP as soon as possible if their child is over a year old and has never had the MMR jab.
Two doses of the MMR vaccine are required to provide the greatest protection against measles.
HPA figures show that the number of toddlers getting the MMR vaccination is climbing steadily, but is still far from the 95% uptake rate needed to stop the spread of the disease in the community.
In December 2010, 89.4% of two-year-old children in the UK had received their first dose of the MMR vaccine.
For five-year-olds, the uptake rate had risen to 92.8%.
The vaccination rate had been well below 95% for several years, ever since The Lancet published controversial research about the MMR vaccine in 1998.
The study has since been discredited, but confidence in the combined measles, mumps and rubella vaccine has been slow to return.
In Scotland, there have been 12 confirmed cases of measles between January and April, compared with no cases at all for the same time last year.
Since the start of 2011 Northern Ireland has had one confirmed case of measles.
In France, the figure of 7,000 cases so far this year already exceeds the 5,090 recorded in the whole of 2010.
The World Health Organization said France was taking immediate steps to control the outbreaks by vaccinating infants at nine months and offering the vaccine to all unimmunised or under-immunised people over that age.
Other European countries reporting an increase in cases of measles are Belgium, Denmark, Finland, Germany, Norway, Romania, the Russian Federation, Sweden and Switzerland.