Urgent action is needed to stop further outbreaks of measles across towns and cities, health experts say.
Vaccination rates across the country have been falling, but there are particular concerns about some regions, including parts of London and the West Midlands.
The UK Health Security Agency (UKHSA) is calling for more people to get the MMR jab to prevent the spread of the potentially deadly virus.
Official figures show there have been 216 confirmed and 103 probable measles cases in the West Midlands since last October.
Around 80% of cases were identified in Birmingham and about 10% in Coventry, with the majority being in children aged under 10.
The UKHSA has declared a national incident signalling the growing public health risk and to enable the organisation to focus on limiting the outbreak, including targeting those at greatest risk.
Chief executive Dame Professor Jenny Harries has stressed the need for all areas to turn around falling vaccination rates across the country.
She said: “Colleagues across the West Midlands have worked tirelessly to try to control the outbreak, but with vaccine uptake in some communities so low, there is now a very real risk of seeing the virus spread in other towns and cities.
“Children who get measles can be very poorly and some will suffer life changing complications.
“The best way for parents to protect their children from measles is the MMR vaccine. Two doses of the MMR vaccine give lifelong protection and it’s never too late to catch up.”
She added: “Immediate action is needed to boost MMR uptake across communities where vaccine uptake is low.
“We need a long-term concerted effort to protect individuals and to prevent large measles outbreaks.”
Measles spreads very easily among those who are unvaccinated, especially in nurseries and schools.
The most serious cases can lead to hospitalisation and even death.
People in certain vulnerable groups including babies and young children, pregnant women, and people with weakened immunity, are at increased risk of complications from measles.
More than 99% of those who have two doses of the MMR vaccine will be protected against measles and rubella.
The vaccine also provides protection against mumps.