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.
more BBC News - Measles outbreak prompts plea to vaccinate children
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.
more BBC News - Measles outbreak prompts plea to vaccinate children